Method of making nanoscale particles of AZO pigments in a microreactor or micromixer

ABSTRACT

A process for preparing nanoscale azo pigment particles includes providing an organic pigment precursor that contains at least one functional moiety, providing a sterically bulky stabilizer compound that contains at least one functional group, and carrying out a chemical reaction to form a pigment composition in a microreactor or micromixer, whereby the functional moiety found on the pigment precursor is incorporated within the pigment and non-covalently associated with the functional group of the stabilizer, so as to allow the formation of nanoscale-sized pigment particles and the production of such in a microreactor under laminar or turbulent flow conditions without clogging.

TECHNICAL FIELD

This disclosure is generally directed to methods for producing nanoscalepigment particle compositions. More specifically, this disclosure isdirected to methods of making organic nanoscale azo pigments in amicroreactor or micromixer. Such particles are useful, for example, asnanoscopic colorants for such compositions as inks, toners and the like.

CROSS-REFERENCE TO RELATED APPLICATIONS

Disclosed in commonly assigned U.S. patent application Ser. No.11/759,913 to Rina Carlini et al. filed Jun. 7, 2007, is a nanoscalepigment particle composition, comprising: an organic monoazo lakedpigment including at least one functional moiety, and a sterically bulkystabilizer compound including at least one functional group, wherein thefunctional moiety associates non-covalently with the functional group;and the presence of the associated stabilizer limits the extent ofparticle growth and aggregation, to afford nanoscale-sized pigmentparticles. Also disclosed is a process for preparing nanoscale-sizedmonoazo laked pigment particles, comprising: preparing a first reactionmixture comprising: (a) a diazonium salt including at least onefunctional moiety as a first precursor to the laked pigment and (b) aliquid medium containing diazotizing agents generated in situ fromnitrous acid derivatives; and preparing a second reaction mixturecomprising: (a) a coupling agent including at least one functionalmoiety as a second precursor to the laked pigment and (b) a stericallybulky stabilizer compound having one or more functional groups thatassociate non-covalently with the coupling agent; and (c) a liquidmedium combining the first reaction mixture into the second reactionmixture to form a third solution and effecting a direct couplingreaction which forms a monoazo laked pigment composition wherein thefunctional moiety associates non-covalently with the functional groupand having nanoscale particle size. Further disclosed is a process forpreparing nanoscale monoazo laked pigment particles, comprising:providing a monoazo precursor dye to the monoazo laked pigment thatincludes at least one functional moiety; subjecting the monoazoprecursor dye to an ion exchange reaction with a cation salt in thepresence of a sterically bulky stabilizer compound having one or morefunctional groups; and precipitating the monoazo laked pigment asnanoscale particles, wherein the functional moiety of the pigmentassociates non-covalently with the functional group of the stabilizerand having nanoscale particle size.

Disclosed in commonly assigned U.S. patent application Ser. No.11/933,471 to Rina Carlini et al. filed Nov. 1, 2007, is a process forpreparing nanoscale particles of monoazo laked pigments, comprising:providing an organic pigment precursor to a monoazo laked pigment thatcontains at least one functional moiety, providing a sterically bulkystabilizer compound that contains at least one functional group, andcarrying out a chemical reaction to form a monoazo laked pigmentcomposition, whereby the functional moiety found on the pigmentprecursor is incorporated within the monoazo laked pigment andnon-covalently associated with the functional group of the stabilizer,so as to allow the formation of nanoscale-sized pigment particles. Inone embodiment, the process comprises preparing a first reaction mixturecomprising: (a) a diazonium salt including at least one functionalmoiety as a first precursor to the laked pigment and (b) a liquid mediumcontaining diazotizing agents; preparing a second reaction mixturecomprising: (a) a coupling agent including at least one functionalmoiety as a second precursor to the laked pigment and (b) a stericallybulky stabilizer compound having one or more functional groups thatassociate non-covalently with the coupling agent; and (c) a liquidmedium; combining the first reaction mixture into the second reactionmixture to form a third solution; and effecting a direct couplingreaction which forms a monoazo laked pigment composition havingnanoscale particle size, and wherein a functional moiety of the pigmentassociates non-covalently with the functional group of the stabilizer.In another embodiment the process comprises providing a monoazoprecursor dye to the monoazo laked pigment that includes at least onefunctional moiety; subjecting the monoazo precursor dye to an ionexchange reaction with a cation in the presence of a sterically bulkystabilizer compound having one or more functional groups; andprecipitating the monoazo laked pigment having nanoscale particle size,wherein the functional moiety of the pigment associates non-covalentlywith the functional group of the stabilizers.

Disclosed in commonly assigned U.S. patent application Ser. No.11/759,906 to Maria Birau et al. filed Jun. 7, 2007, is a nanoscalepigment particle composition, comprising: a quinacridone pigmentincluding at least one functional moiety, and a sterically bulkystabilizer compound including at least one functional group, wherein thefunctional moiety associates non-covalently with the functional group;and the presence of the associated stabilizer limits the extent ofparticle growth and aggregation, to afford nanoscale-sized particles.Also disclosed is a process for preparing nanoscale quinacridone pigmentparticles, comprising: preparing a first solution comprising: (a) acrude quinacridone pigment including at least one functional moiety and(b) a liquid medium; preparing a second solution comprising: (a) asterically bulky stabilizer compound having one or more functionalgroups that associate non-covalently with the functional moiety, and (b)a liquid medium; combining the first reaction mixture into the secondreaction mixture to form a third solution and effecting a directcoupling reaction which forms a quinacridone pigment composition whereinthe functional moiety associates non-covalently with the functionalgroup and having nanoscale particle size.

Disclosed in commonly assigned U.S. patent application Ser. No.12/044,613 to Rina Carlini filed Mar. 7, 2008, is a nanoscale pigmentparticle composition, comprising: a benzimidazolone pigment, and asterically bulky stabilizer compound associated non-covalently with thebenzimidazolone pigment; wherein the presence of the associatedstabilizer limits an extent of particle growth and aggregation, toafford nanoscale-sized pigment particles. Also disclosed is a processfor preparing nanoscale particles of benzimidazolone pigments,comprising: providing one or more organic pigment precursor to abenzimidazolone pigment, providing a solution or suspension of asterically bulky stabilizer compound that associates non-covalently withthe benzimidazolone moiety on one of the pigment precursors, andcarrying out a chemical coupling reaction to form a benzimidazolonepigment composition, whereby the pigment precursors are incorporatedwith the benzimidazolone pigment and one or more functional moieties onthe benzimidazolone pigment is non-covalently associated with the stericstabilizer, so as to limit the extent of particle growth and aggregationand result in nanoscale-sized pigment particles.

The entire disclosure of the above-mentioned applications are totallyincorporated herein by reference.

BACKGROUND

A printing ink is generally formulated according to strict performancerequirements demanded by the intended market application and requiredproperties. Whether formulated for office printing or for productionprinting, a particular ink is expected to produce images that are robustand durable under stress conditions. In a typical design of apiezoelectric ink jet printing device, the image is applied by jettingappropriately colored inks during four to six rotations (incrementalmovements) of a substrate (an image receiving member or intermediatetransfer member) with respect to the ink jetting head, i.e., there is asmall translation of the printhead with respect to the substrate inbetween each rotation. This approach simplifies the printhead design,and the small movements ensure good droplet registration. At the jetoperating temperature, droplets of liquid ink are ejected from theprinting device and, when the ink droplets contact the surface of therecording substrate, either directly or via an intermediate heatedtransfer belt or drum, they quickly solidify to form a predeterminedpattern of solidified ink drops.

Pigments are a class of colorants useful in a variety of applicationssuch as, for example, paints, plastics and inks, including inkjetprinting inks. Dyes have typically been the colorants of choice forinkjet printing inks because they are readily soluble colorants whichenable jetting of the ink. Dyes have also offered superior and brilliantcolor quality with an expansive color gamut for inks, when compared toconventional pigments. However, since dyes are molecularly dissolved inthe ink vehicle, they are often susceptible to unwanted interactionsthat lead to poor ink performance, for example photooxidation from light(will lead to poor lightfastness), dye diffusion from the ink into paperor other substrates (will lead to poor image quality and showthrough),and the ability for the dye to leach into another solvent that makescontact with the image (will lead to poor water-/solvent-fastness). Incertain situations, pigments are the better alternative as colorants forinkjet printing inks since they are insoluble and cannot be molecularlydissolved within the ink matrix, and therefore do not experiencecolorant diffusion. Pigments can also be significantly less expensivethan dyes, and so are attractive colorants for use in all printing inks.

Key challenges with using pigments for inkjet inks are their largeparticle sizes and wide particle size distribution, the combination ofwhich can pose critical problems with reliable jetting of the ink (i.e.inkjet nozzles are easily blocked). Pigments are rarely obtained in theform of single crystal particles, but rather as large aggregates ofcrystals and with wide distribution of aggregate sizes. The colorcharacteristics of the pigment aggregate can vary widely depending onthe aggregate size and crystal morphology. Thus, an ideal colorant thatis widely applicable in, for example, inks and toners, is one thatpossesses the best properties of both dyes and pigments, namely: 1)superior coloristic properties (large color gamut, brilliance, hues,vivid color); 2) color stability and durability (thermal, light,chemical and air-stable colorants); 3) minimal or no colorant migration;4) processable colorants (easy to disperse and stabilize in a matrix);and 5) inexpensive material cost. Thus, there is a need addressed byembodiments of the present invention, for smaller nano-sized pigmentparticles that minimize or avoid the problems associated withconventional larger-sized pigment particles. There further remains aneed for processes for making and using such improved nano-sized pigmentparticles as colorant materials. The present nanosized pigment particlesare useful in, for example, paints, coatings and inks (e.g., inkjetprinting inks) and other compositions where pigments can be used such asplastics, optoelectronic imaging components, photographic components,and cosmetics among others.

Microreactors have been defined as “Microsystems fabricated, at leastpartially, by methods of microtechnology and precision engineering.Fluid channels range from 1 um (nanoreactors) to 1 mm (minireactors).”See Microreactors, Ehrfeld, Hessel & Lowe 2000, and W. Ehrfeld et al.,“Microreactors—New Technology for Modern Chemistry, 1^(st) Edition,Wiley-VCH, Weinheim, 5-11 (2001), the entire disclosures of which areincorporated herein by reference. Typical microreactors consist ofminiaturized channels, often imbedded in a flat surface referred to asthe “chip.” These flat surfaces can be glass plates or plates of metalssuch as stainless steel or Hastelloy. Microreactors have proven to behighly valuable tools in organic chemistry due to their wide flexibilityof operating conditions with efficient heat transfer, optimized mixing,and high reaction control. Advantages of a microreactor over moreconventional batch reactions include: faster efficient mixing,selectivity enhanced-side products and secondary reactions reduced,higher yields and purities, extreme reaction conditions, time and costsavings, and increased surface area to volume ratio that results in goodmass and heat transfer. Microreactors are particularly useful for rapidoptimization, screening different reaction conditions, catalysts,ligands, bases, and solvents; mechanistic studies; cost effectiveindustrial scale up; and rapid screening for new pharmaceuticals.Although microreactors have distinct advantages over conventional batchreaction techniques, microreactor chemistry also has its ownshortcomings. For example, microreactors generally do not tolerateparticulate matter well, often clogging.

Few examples exist where micro reaction technology has been applied tothe production of suspensions containing solid materials. This is onaccount of the high potential for blockage of the micro channels thatform these micro devices (less than 1 mm). Some example do exist wherecustom fabricated microreactors can be applied to the synthesis of solidmaterials (pigments). See, for example, U.S. Pat. Nos. 6,437,104 B1,6,469,147 B2, and 6,723,138 B2. Custom made micro fluidic devices havealso been applied for the production of fine pigments. See, for example,U.S. Patent Publications Nos. 2005/0109240 A1 and 2008/0078305 A1. Thesereferences have generally avoided the clogging of microreactor channelsby either ensuring turbulent flow conditions or by designing simplifiedmicrofluidic devices with a limited number of passes (once through) andtherefore a limited number of bends. Neither of these approaches ishowever convenient for the general production of materials inmicroreactors. Turbulent flow conditions require that fluids be pumpedthrough the microreactor at high flowrates and this leads to highpressure drops through the system that increase the required pumpdelivery pressures. Secondly high flowrates may be detrimental to thesynthesis of the desired material as the material is degraded by flowinduced shear during the synthesis. Simplified microfluidic devices witha single straight through pass are also not convenient as such devicesoffer limited mixing efficiencies and therefore low yields. Furthermorethe productivity of such devices is restricted by the limited residencetimes they offer. Low flowrates in the range of μL/min are generallyused to make materials with these devices. Such flowrates are notpractical for the large scale production of solid materials.Microreactors with flow rates that are 3 to 6 orders of magnitude higherare more practical (mL/min to L/min). A more desirable microreactorprocess for solid material synthesis, and in our specific case pigmentsynthesis, would be one that offers a wide range of possible residencetimes through the use of multiple bends and passes internal to themicroreactor and under flow conditions that are laminar withoutclogging. No such process has been reported. We report here amicroreactor process that produces solid pigment particles under laminarflow conditions and at high flowrates by limiting the growth of theformed pigment particles to the nanometer range and thus preventingclogging. This process leads to a production rate of the desirednanopigment and permits good control of the reaction conditions ascompared to the conventional batch process.

The following documents provide background information:

U.S. Patent Application Publication No. 2008/0078305 A1 describes two ormore solutions comprising an organic pigment solution in which anorganic pigment is dissolved in a good solvent, and a poor solventcompatible with the good solvent, or a solution of the poor solvent areallowed to flow through a microchannel in a non-laminar state; andorganic pigment fine particles are deposited from the organic pigmentsolution in a course of flowing through the microchannel by changing thesolubility of the organic pigment solution with the poor solvent or thesolution of the poor solvent. As a result, nanometer-scale monodisperseorganic pigment fine particles can be produced in a stable manner.

U.S. Patent Application Publication No. 2006/0194897 A1 describes apigment dispersion that can be suitably used as a coloring material forinks, especially inks for ink-jet recording, comprising a coloredpigment in primary particles dispersed stably in a liquid medium, and aprocess for producing the pigment dispersion. A pigment dispersioncomprising a colored pigment that is substantially of a primary particlemaintaining type and is dispersed in a liquid medium, a process forproducing the pigment dispersion, and an ink and recorded image usingthe pigment dispersion.

U.S. Patent Application Publication No. 2003/0164118 A1 describes aprocess for conditioning organic pigments by introducing a liquidprepigment suspension into a miniaturized continuous reactor andthermally treating therein.

U.S. Patent Application Publication No. 2003/0158410 A1 describes aprocess for preparing diketopyrrolopyrrole pigments comprises conductingthe elementary steps of pigment synthesis (reaction and hydrolysis) in aminiaturized continuous reactor.

U.S. Patent Application Publication No. 2007/0289500 A1 describes amethod of producing a dispersion of a pigment, comprising: bringing asolution in which an organic pigment is dissolved, and an aqueousmedium, into contact with each other in a channel having an equivalentdiameter of 1 mm or less, thereby making the pigment into a fineparticle thereof, wherein at least one of the solution and the aqueousmedium comprises at least one anionic dispersing agent.

U.S. Patent Application Publication No. 2007/0213516 A1 describes aprocess for producing high-purity azo colorants is characterized in that(a) at least the azo coupling is carried out in a micro-reactor, (b) theazo-dye produced in the micro-reactor is brought into intimate contactwith an organic solvent from the group of the C₃-C₆ alcohols, the C₄-C₁₀ether alcohols and the halogenated aromatics at a temperature from 0 to60° C., and (c) the azo dye produced in the micro-reactor is subjectedto membrane purification in an aqueous or solvent-containing suspension.

WO 2006/132443 A1 describes a method of producing organic pigment fineparticles, wherein when producing organic pigment fine particles byallowing two or more solutions at least one of which is an organicpigment solution in which an organic pigment is dissolved to flowthrough a microchannel, the organic pigment solution flows through themicrochannel in a non-laminar state. Accordingly, the contact area ofsolutions per unit time can be increased and the length of diffusionmixing can be shortened, and thus instantaneous mixing of solutionsbecomes possible. As a result, nanometer-scale monodisperse organicpigment fine particles can be produced in a stable manner.

U.S. Pat. No. 7,160,380 describes a method of producing a fine particleof an organic pigment, containing the steps of: flowing a solution of anorganic pigment dissolved in an alkaline or acidic aqueous medium,through a channel which provides a laminar flow; and changing a pH ofthe solution in the course of the laminar flow.

U.S. Pat. No. 7,262,284 describes a method for the production of a diazopigment, or a mixture of diazo pigments, according to formula (1) of thespecification by azo coupling, wherein the azo coupling product issubjected to a finish in an organic solvent or in an aqueous organicsolvent with a neutral or alkaline pH.

U.S. Patent Application Publication No. 2002/0058794 A1 describes aprocess for preparing disazo condensation pigments by diazotization ofan aromatic amine, azo coupling with a coupling component to form anazocarboxylic acid or azodicarboxylic acid, formation of an azocarbonylchloride or azodicarbonyl dichloride and condensation of the azocarbonylchloride with an aromatic diamine or of the azodicarbonyl dichloridewith an aromatic amine comprises effecting the acyl chloride formationand/or the condensation and optionally the diazotization and optionallythe azo coupling in a microreactor.

U.S. Patent Application Publication No. 2001/0029294 A1 describes azocolorants that are prepared by conducting the diazotization of aromaticor heteroaromatic amines or the azo coupling reaction or thediazotization and the azo coupling reaction in a microreactor.

Hideki Maeta et al., “New Synthetic Method of Organic Pigment NanoParticle by Micro Reactor System,” in an abstract available on theinternet, describes a new synthetic method of an organic pigmentnanoparticle was realized by micro reactor. A flowing solution of anorganic pigment, which dissolved in an alkaline aqueous organic solvent,mixed with a precipitation medium in a micro channel. Two types of microreactor can be applied efficiently on this build-up procedure withoutblockage of the channel. The clear dispersion was extremely stable andhad narrow size distribution, which were the features, difficult torealize by the conventional pulverizing method (breakdown procedure).These results proved the effectiveness of this process on micro reactorsystem.

U.S. Patent Application Publication No. 2005/0109240 describes a methodof producing a fine particle of an organic pigment, containing the stepsof: flowing a solution of an organic pigment dissolved in an alkaline oracidic aqueous medium, through a channel which provides a laminar flow;and changing a pH of the solution in the course of the laminar flow.

WO 2006/132443 A1 describes a method of producing organic pigment fineparticles by allowing two or more solutions, at least one of which is anorganic pigment solution in which an organic pigment is dissolved, toflow through a microchannel, the organic pigment solution flows throughthe microchannel in a non-laminar state. Accordingly, the contact areaof solutions per unit time can be increased and the length of diffusionmixing can be shortened, and thus instantaneous mixing of solutionsbecomes possible. As a result, nanometer-scale monodisperse organicpigment fine particles can be produced in a stable manner.

K. Balakrishnan et al., “Effect of Side-Chain Substituents onSelf-Assembly of Perylene Diimide Molecules: Morphology Control,” J. Am.Chem. Soc., vol. 128, p. 7390-98 (2006) describes the use ofcovalently-linked aliphatic side-chain substituents that werefunctionalized onto perylene diimide molecules so as to modulate theself-assembly of molecules and generate distinct nanoparticlemorphologies (nano-belts to nano-spheres), which in turn impacted theelectronic properties of the material. The side-chain substituentsstudied were linear dodecyl chain, and a long branched nonyldecyl chain,the latter substituent leading to the more compact, sphericalnanoparticle.

U.S. Patent Application Publication No. 2003/0199608 discloses afunctional material comprising fine coloring particles having an averageprimary particle diameter of 1 to 50 nm in a dried state, and having aBET specific surface area value of 30 to 500 m²/g and a lighttransmittance of not less than 80%. The functional material composed offine coloring particles, exhibits not only an excellent transparency butalso a high tinting strength and a clear hue.

U.S. Pat. No. 6,537,364 discloses a process for the fine division ofpigments which comprises dissolving coarsely crystalline crude pigmentsin a solvent and precipitating them with a liquid precipitation mediumby spraying the pigment solution and the precipitation medium throughnozzles to a point of conjoint collision in a reactor chamber enclosedby a housing in a microjet reactor, a gas or an evaporating liquid beingpassed into the reactor chamber through an opening in the housing forthe purpose of maintaining a gas atmosphere in the reactor chamber, andthe resulting pigment suspension and the gas or the evaporated liquidbeing removed from the reactor through a further opening in the housingby means of overpressure on the gas entry side or underpressure on theproduct and gas exit side.

U.S. Pat. No. 5,679,138 discloses a process for making ink jet inks,comprising the steps of: (A) providing an organic pigment dispersioncontaining a pigment, a carrier for the pigment and a dispersant; (B)mixing the pigment dispersion with rigid milling media having an averageparticle size less than 100 μm; (C) introducing the mixture of step (B)into a high speed mill; (D) milling the mixture from step (C) until apigment particle size distribution is obtained wherein 90% by weight ofthe pigment particles have a size less than 100 nanometers (nm); (E)separating the milling media from the mixture milled in step (D); and(F) diluting the mixture from step (E) to obtain an ink jet ink having apigment concentration suitable for ink jet printers.

Japanese Patent Application Publications Nos. JP 2007023168 and JP2007023169 discloses providing a pigment dispersion compound excellentin dispersibility and flowability used for the color filter which hashigh contrast and weatherability. The solution of the organic material,for example, the organic pigment, dissolved in a good solvent under theexistence of alkali soluble binder (A) which has an acidic group, and apoor solvent which makes the phase change to the solvent are mixed. Thepigment nanoparticles dispersed compound re-decentralized in the organicsolvent containing the alkali soluble binder (B) which concentrates theorganic pigment nanoparticles which formed the organic pigment as theparticles of particle size less than 1 μm, and further has the acidicgroup.

Kazuyuki Hayashi et al., “Uniformed nano-downsizing of organic pigmentsthrough core-shell structuring,” Journal of Materials Chemistry, 17(6),527-530 (2007) discloses that mechanical dry milling of organic pigmentsin the presence of mono-dispersed silica nanoparticles gave core-shellhybrid pigments with uniform size and shape reflecting those of theinorganic particles, in striking contrast to conventional milling as abreakdown process, which results in limited size reduction and wide sizedistribution.

U.S. Patent Application Publication No. 2007/0012221 describes a methodof producing an organic pigment dispersion liquid, which has the stepsof: providing an alkaline or acidic solution with an organic pigmentdissolved therein and an aqueous medium, wherein a polymerizablecompound is contained in at least one of the organic pigment solutionand the aqueous medium; mixing the organic pigment solution and theaqueous medium; and thereby forming the pigment as fine particles; thenpolymerizing the polymerizable compound to form a polymer immobile fromthe pigment fine particles.

The appropriate components and process aspects of each of the foregoingmay be selected for the present disclosure in embodiments thereof, andthe entire disclosure of the above-mentioned references are totallyincorporated herein by reference.

SUMMARY

The present disclosure addresses these and other needs, by providingmethods for producing nanoscale pigment particle compositions. Thedisclosure provides a process for making nanoscale organic pigmentparticles such as, for example, azo type pigments like azo pigments andazo laked pigments, including monoazo, disazo, and the like, using acommercial microreactor or micromixer, and describes an operating windowwithin which this process operates for the formation of suchnanoparticles. Production rates with this process are approximately fivetimes higher than those achieved in batch. Under laminar flow conditionsthis process allows the continuous production of nanopigments withoutclogging of the microreactor or micromixer. The process of nanoscalepigment production is not limited to the laminar flow regime but canalso be practiced in the turbulent flow regime. In addition, thenanoscale organic azo pigments produced by using this microreactor ormicromixer process operating provides particles with enhanced coloristicproperties.

In an embodiment, the present disclosure provides a process forpreparing nanoscale azo pigment particles, comprising: providing anorganic pigment precursor that contains at least one functional moiety,

providing a sterically bulky stabilizer compound that contains at leastone functional group, and

carrying out a chemical reaction to form a pigment composition in amicroreactor or micromixer, whereby the functional moiety found on thepigment precursor is incorporated within the pigment and non-covalentlyassociated with the functional group of the stabilizer, so as to allowthe formation of nanoscale-sized pigment particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary diagram of the process of the disclosure.

FIGS. 2 a-2 b show TEM images of pigment particles prepared according tothe Examples and Comparative Examples.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the present disclosure provide improved methods forproducing nanoscale pigment particle compositions in a microreactor ormicromixer. The nanoscale pigment particle compositions generallycomprise an organic pigment, such as of the azo type, including at leastone functional moiety that associates non-covalently with a functionalgroup from a sterically bulky stabilizer compound. As used herein “azotype” refers to azo or azo laked pigments, where the azo group can bemonoazo, disazo, or the like. The presence of the associated stabilizerlimits the extent of particle growth and aggregation, to affordnanoscale particles. This approach of using steric stabilizers permitsthe controlled assembly and production of stabilized nanopigments inmicroreactors or micromixers without clogging under both laminar andturbulent flow conditions.

Organic monoazo “laked” pigments are the insoluble metal salt colorantsof monoazo colorants which can include monoazo dyes or pigments, and incertain geographic regions these pigments have been referred to aseither “toners” or “lakes.” Other “laked” pigments are similarly metalsalt forms of the underlying colorant. The process of ion complexationwith a metal salt, or “laking” process, provides decreased solubility ofthe non-ionic monoazo pigment, which can enhance the migrationresistance and thermal stability properties of a monoazo pigment, andthereby enable the applications of such pigments for robust performance,such as colorizing plastics and heat-stable paints for outdoor use. Ageneral representation of both monoazo and monoazo-laked pigments isshown in the general formula 1, which such pigments are structurallycomprised of a diazo group (denoted G_(d)) and a nucleophilic couplinggroup (denoted as G_(c)) that are linked together with one azo (N═N)functional group. In the case of laked monoazo pigments, which are ionicsalt compounds, the cation M^(n+) is shown as being coordinated to oneor more ionic functional moieties on the pigments and is typically ametal salt. Either or both of the groups G_(d) and G_(c) can contain oneor more ionic functional moieties (denoted as FM), such as sulfonate orcarboxylate anions or the like.

As an example, the organic monoazo laked pigment PR 57:1 (“PR” refers toPigment Red) has two functional moieties of two different types, asulfonate anion group (SO₃ ⁻) and carboxylate anion group (CO₂ ⁻) and ametal counter-cation M^(n+) that is chosen from Group 2 alkaline earthmetals such as Ca²⁺. Other monoazo laked pigment compositions also existthat have a counter-cation chosen from either Group 2 alkaline earthmetals (Be, Mg, Ca, Sr, Ba,), Group 3 metals (B, Al, Ga), Group 1 alkalimetals(Li, Na, K, Cs), the transition metals such as Cr, Mn, Fe, Ni, Cu,Zn, or others non-metallic cations such as ammonium (NR₄ ⁺), phosphonium(PR₄ ⁺) wherein R-group can be H or alkyl group having from about 1 toabout 12 carbons. Further, the azo group in the compounds can generallyassume one or more tautomeric forms, such as the “azo” tautomer formwhich has the (N═N) linkage, and the “hydrazone” tautomer form which hasthe (C═N—NH—) linkage that is stabilized by an intramolecular hydrogenbond, where the hydrazone tautomer is known to be the preferredstructural form for PR 57:1.

It is also understood that formula (1) denotes both such tautomer forms.Due to the structural nature of monoazo laked pigments being ionicsalts, it is possible to have compounds that associate non-covalentlywith the pigment, such as organic or inorganic ionic compounds that canassociate directly through ionic or coordination-type bonding, andtypically with the counter-cation group like M^(n+). Such ioniccompounds are included in a group of compounds which herein are referredto as “stabilizers”, and that function to reduce the surface tension ofthe pigment particle and neutralize attractive forces between two ormore pigment particles or structures, thereby stabilizing the chemicaland physical structure of the pigment.

The term “precursor” as used in “precursor to the organic pigment” canbe any chemical substance that is an advanced intermediate in the totalsynthesis of a compound (such as the organic pigment). In embodiments,the organic pigment and the precursor to the organic pigment may or maynot have the same functional moiety. In embodiments, the precursor tothe organic pigment may or may not be a colored compound. In still otherembodiments, the precursor and the organic pigment can have differentfunctional moieties. In embodiments, where the organic pigment and theprecursor have a structural feature or characteristic in common, thephrase “organic pigment/pigment precursor” is used for conveniencerather than repeating the same discussion for each of the organicpigment and the pigment precursor.

The term “complementary” as used in “complementary functional moiety” ofthe stabilizer indicates that the complementary functional moiety iscapable of noncovalent chemical bonding with the functional moiety ofthe organic pigment and/or the functional moiety of a pigment precursor.

The functional moiety (denoted as FM) of the organic pigment/precursorcan be any suitable moiety capable of non-covalent bonding with thecomplementary functional group of the stabilizer. Illustrativefunctional moieties of the organic pigment/precursor include (but arenot limited to) the following: sulfonate/sulfonic acid,(thio)carboxylate/(thio)carboxylic acid, phosphonate/phosphonic acid,ammonium and substituted ammonium salts, phosphonium and substitutedphosphonium salts, substituted carbonium salts, substituted aryliumsalts, alkyl/aryl (thio)carboxylate esters, thiol esters, primary orsecondary amides, primary or secondary amines, hydroxyl, ketone,aldehyde, oxime, hydroxylamino, enamines (or Schiff base), porphyrins,(phthalo)cyanines, urethane or carbamate, substituted ureas, guanidinesand guanidinium salts, pyridine and pyridinium salts, imidazolium and(benz)imidazolium salts, (benz)imidazolones, pyrrolo, pyrimidine andpyrimidinium salts, pyridinone, piperidine and piperidinium salts,piperazine and piperazinium salts, triazolo, tetraazolo, oxazole,oxazolines and oxazolinium salts, indoles, indenones, and the like.

Pigment precursors for making both monoazo and monoazo lakednanopigments consist of a substituted aniline precursor (denoted as “DC”in Table 1) which forms the diazo group G_(d) of Formula (1), anucleophilic or basic coupling compound (denoted as “CC” in Tables 2-6)which leads to the coupling group G. of Formula (1), and for monoazolaked pigments specifically, a cation salt is also present which ispreferably a metal (denoted as “M” as shown in Formula (1)).Representative examples of the aniline precursor of monoazo and lakedmonoazo pigments that have the functional moiety capable of non-covalentbonding with a complementary functional group on the stabilizer, include(but are not limited to) the following structures (with the functionalmoiety “FM” denoted, if applicable).

In an embodiment, the substituted aniline precursor (DC) which leads tothe diazonium group can be of the formula (2):

where R₁, R₂, and R₃ independently represent H, a straight or branchedalkyl group of from about 1 to about 10 carbon atoms (such as methyl,ethyl, propyl, butyl, and the like), halogen (such as Cl, Br, I), NH₂,NO₂, CO₂H, CH₂CH₃, and the like; and functional moiety FM representsSO₃H, —C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted orsubstituted with either halogens (such as Cl, Br, I, F) or alkyl groupshaving from about 1 to about 10 carbons (such as methyl, ethyl, propyl,butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂,and the like. The substituted aniline precursor (DC) can be also beTobias Acid, of the formula (3):

Specific examples of types of aniline precursors (DC) that are used tomake the diazo group G_(d) in the monoazo and laked monoazo pigmentsinclude those of Table 1:

TABLE 1

Precursor to Functional Group G_(d) Moiety FM R₁ R₂ R₃ DC1 SO₃H CH₃ HNH₂ DC2 SO₃H CH₃ Cl NH₂ DC3 SO₃H Cl CH₃ NH₂ DC4 SO₃H Cl CO₂H NH₂ DC5SO₃H Cl CH₂CH₃ NH₂ DC6 SO₃H Cl Cl NH₂ DC7 SO₃H H NH₂ H DC8 SO₃H H NH₂CH₃ DC9 SO₃H NH₂ H Cl DC10 SO₃H H H NH₂ DC11 SO₃H H NH₂ H DC12 SO₃H NO₂NH₂ H DC13

NH₂ CH₃ H DC14 CO₂H H H NH₂ DC15 Cl H H NH₂ DC16 NH₂ CH₃ H H DC17 NH₂ HCH₃ H DC18

NH₂ CH₃ H DC19

H NH₂ H DC20 NH₂ H H H DC21

DC22 SO₂NHCH₃ OCH₃ NH₂ CH₃ DC23 CO₂CH₃ H H NH₂

In an embodiment, the coupling group G_(c) of Formula (1) can includeβ-naphthol and derivatives of Formula (4), naphthalene sulfonic acidderivatives of Formulas (5) and (6), pyrazolone derivatives of Formula(7), acetoacetic arylide derivatives of Formula (8), and the like. Informulas (4)-(8), the asterisk “*” denotes the point of coupling orattachment to the monoazo (N═N) linkage.

where FM represents H, CO₂H, SO₃H, —C(═O)—NH-Aryl-SO₃ ⁻ where the arylgroup can be unsubstituted or substituted with either halogens (such asCl, Br, I, F) or alkyl groups having from about 1 to about 10 carbons(such as methyl, ethyl, propyl, butyl and the like) CO₂H, halogen (suchas Cl, Br, I), NH₂, —C(═O)—NH₂, substituted benzamides such as:

wherein groups R₂′ R₃′, R₄′ and R₅′ can independently be H, alkyl groupshaving from about 1 to 10 carbons (such as methyl, ethyl, propyl, butyl,and the like), alkoxyl groups (such as OCH₃, OCH₂CH₃, and the like),hydroxyl or halogen (such as Cl, Br, I, F) or nitro NO₂; orbenzimidazolone amides such as:

and the like.

where FM represents preferably SO₃H, but also can represent CO₂H,—C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted orsubstituted with either halogens (such as Cl, Br, I, F) or alkyl groupshaving from about 1 to about 10 carbons (such as methyl, ethyl, propyl,butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂groups R₃ and R₄ independently represent H, SO₃H, and the like.

where FM represents preferably SO₃H, but also can represent CO₂H,—C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted orsubstituted with either halogens (such as Cl, Br, I, F) or alkyl groupshaving from about 1 to about 10 carbons (such as methyl, ethyl, propyl,butyl and the like) CO₂H, halogen (such as Cl, Br, I), NH₂, —C(═O)—NH₂;R₁, R₂, R₃ and R₄ independently represent H, SO₃H, —C(═O)—NH-Phenyl, andthe like.

where G represents CO₂H, straight or branched alkyl such as having from1 to about 10 carbons atoms (such as methyl, ethyl, propyl, butyl, orthe like), and the like; and R₁′, R₂′, R₃′ and R₄′ independentlyrepresent H, halogen (such as Cl, Br, I), SO₃H, nitro (NO₂) or alkoxylgroup such as OCH₃ or OCH₂CH₃ and the like.

where R₁′ represents a straight or branched alkyl group having, forexample, from 1 to about 10 carbon atoms (such as methyl, ethyl, propyl,butyl, and the like); R₂′ represents a benzimidazolone group:

or a substituted aryl group

where each of R_(a), R_(b), and R_(c) independently represents H, astraight or branched alkyl group having, for example, from 1 to about 10carbon atoms (such as methyl, ethyl, propyl, butyl, and the like),alkoxyl groups such as OCH₃ or OCH₂CH₃ and the like, halogen (such asCl, Br, I), nitro NO₂, and the like.

Representative examples of the nucleophilic coupling component as aprecursor of monoazo and laked monoazo pigments which have thefunctional moiety that is capable of non-covalent bonding with acomplementary functional group on the stabilizer, include (but are notlimited to) the following structures shown in Tables 2-6 (with thefunctional moiety “FM” denoted, if applicable):

TABLE 2

Precursor to Class of Coupling Functional Moiety group G_(c) ComponentFM CC1 β-Naphthol H CC2 β-oxynahthoic acid CO₂H (“BONA”) CC3 Naphthol ASderivatives

CC6 Benzimidazolone

* = point of coupling to diazo component

TABLE 3

Precursor to Class of Coupling group G_(c) Component FM R₃ R₄ CC4aNaphthalene Sulfonic SO₃H H H Acid derivatives CC4b Naphthalene SulfonicSO₃H SO₃H H Acid derivatives * = point of coupling to diazo component

TABLE 4

Precursor to Class of Coupling group G_(c) Component FM R₁ R₂ R₃ R₄ CC5Naphthalene Sulfonic Acid derivatives SO₃H

H H SO₃H * = point of coupling to diazo component

TABLE 5

Precursor to Class of Coupling group G_(c) Component G R₁′ R₂′ R₃′ R₄′CC7 Pyrazolone deriv. CO₂H H H SO₃H H CC8 Pyrazolone deriv. CH₃ H H SO₃HH CC9 Pyrazolone deriv. CH₃ H SO₃H H H CC10 Pyrazolone deriv. CH₃ Cl HSO₃H Cl * = point of coupling to diazo component

TABLE 6

Precursor to Class of Coupling group G_(c) Component R₁′ R₂′ R_(a) R_(b)R_(c) CC11 Acetoacetic arylide CH₃

H H H CC12 Acetoacetic arylide CH₃

CH₃ H H CC13 Acetoacetic arylide CH₃

Cl H H CC14 Acetoacetic arylide CH₃

H OCH₃ H CC15 Acetoacetic benzimidazolne CH₃

— — — * = point of coupling to diazo component

The laked monoazo organic pigments, and in some embodiments, theprecursor of such pigments, also generally include one or morecounterions as part of the overall structure. Such counterions can be,for example, any suitable counterion including those that are well knownin the art. Such counterions can be, for example, cations or anions ofeither metals or non-metals that include N, P, S and the like, orcarbon-based cations or anions. Examples of suitable cations includeions of Ba, Ca, Cu, Mg, Sr, Li, Na, K, Cs, Mn, Cu, Cr, Fe, Ti, Ni, Co,Zn, V, B, Al, Ga, and other metal ions, as well as ammonium andphosphonium cations, mono-, di-, tri-, and tetra-substituted ammoniumand phosphonium cations, where the substituents can be aliphatic alkylgroups, such as methyl, ethyl, butyl, stearyl and the like, as well asaryl groups such as phenyl or benzyl and the like.

Representative examples of azo and azo laked pigments comprised from aselection of substituted aniline precursors (denoted DC), includingTobias Acid, nucleophilic coupling components (denoted as CC) andoptionally metal salts (denoted as M) to provide the counter-cationMW^(n+) of laked pigments as in formula (1), are listed in Table 7.Other pigment structures may also be formed from other combinations ofDC and CC and optionally metal cation salt (M) that are not shown inTable 7.

TABLE 7

Color Index # Color Index G_(d) G_(c) Metal Salt (C.I.) (C.I.) NameLaked Pigment Class precursor precursor M 15500:1 Red 50:1 0-NaphtholLakes DC14 CC1 ½Ba 15510:1 Orange 17 0-Naphthol Lakes DC7 CC1 Ba 15510:2Orange 17:1 0-Naphthol Lakes DC7 CCl ? Al 15525 Red 68 0-Naphthol LakesDC4 CC1 2 Ca 15580 Red 51 0-Naphthol Lakes DC8 CC1 Ba 15585 Red 530-Naphthol Lakes DC3 CC1 2 Na 15585:1 Red 53:1 0-Naphthol Lakes DC5 CC1Ba 15585:3 Red 53:3 0-Naphthol Lakes DC21 CC1 Sr 15602 Orange 460-Naphthol Lakes DC21 CC1 Ba 15630 Red 49 0-Naphthol Lakes DC21 CC1 2 Na15630:1 Red 49:1 0-Naphthol Lakes DC21 CC1 Ba 15630:2 Red 49:20-Naphthol Lakes DC21 CC1 Ca 15630:3 Red 49:3 0-Naphthol Lakes DC21 CC1Sr 15800 Red 64 0-oxynaphthoic acid (BONA) Lakes DC20 CC2 ½Ba 15800:1Red 64:1 0-oxynaphthoic acid (BONA) Lakes DC20 CC2 ½Ca 15800:2 Brown 50-oxynaphthoic acid (BONA) Lakes DC20 CC2 ½Cu 15825:2 Red 58:20-oxynaphthoic acid (BONA) Lakes DC9 CC2 Ca 15825:4 Red 58:40-oxynaphthoic acid (BONA) Lakes DC9 CC2 Mn 15850:1 Red 57:10-oxynaphthoic acid (BONA) Lakes DC1 CC2 Ca 15860:1 Red 52:10-oxynaphthoic acid (BONA) Lakes DC3 CC2 Ca 15860:2 Red 52:20-oxynaphthoic acid (BONA) Lakes DC3 CC2 Mn 15865:1 Red 48:10-oxynaphthoic acid (BONA) Lakes DC2 CC2 Ba 15865:2 Red 48:20-oxynaphthoic acid (BONA) Lakes DC2 CC2 Ca 15865:3 Red 48:30-oxynaphthoic acid (BONA) Lakes DC2 CC2 Sr 15865:4 Red 48:40-oxynaphthoic acid (BONA) Lakes DC2 CC2 Mn 15865:5 Red 48:50-oxynaphthoic acid (BONA) Lakes DC2 CC2 Mg 15867 Red 200 0-oxynaphthoicacid (BONA) Lakes DC5 CC2 Ca 15880:1 Red 63:1 0-oxynaphthoic acid (BONA)Lakes DC21 CC2 Ca 15880:2 Red 63:2 0-oxynaphthoic acid (BONA) Lakes DC21CC2 Mn 15892 Red 151 Naphthol AS Lakes DC10 CC3 (R₂′ = H R₄′ = SO₃H) Ba15910 Red 243 Naphthol AS Lakes DC2 CC3 (R₂′ = OCH₃ R₄′ = H) ½Ba 15915Red 247 Naphthol AS Lakes DC13 CC3 (R₂′ = H R₄′ = SO₃H) Ca 15985:1Yellow 104 Naphthalene Sulfonic Acid Lakes DC7 CC4a ? Al 15990 Orange 19Naphthalene Sulfonic Acid Lakes DC15 CC4a ½Ba 16105 Red 60 NaphthaleneSulfonic Acid Lakes DC14 CC4b 3/2Ba 18000:1 Red 66 Naphthalene SulfonicAcid Lakes DC16 CC5 ½Ba, Na 19140:1 Yellow 100 Pyrazolone Lakes DC7 CC7Al 18792 Yellow 183 Pyrazolone Lakes DC6 CC9 Ca 18795 Yellow 191Pyrazolone Lakes DC2 CC9 Ca — Yellow 190 Pyrazolone Lakes DC6 CC10 Ca13980 Yellow 151 Azo-Benzimidazolone DC14 CC15 none 12513 Red 175Azo-Benzimidazolone DC23 CC15 none 12516 Red 185 Azo-BenzimidazoloneDC22 CC15 none

A stabilizer can be any compound that has the function to reduce thesurface tension of the pigment particle and neutralize attractive forcesbetween two or more pigment particles or structures, thereby stabilizingthe chemical and physical structure of the primary pigment particle. Inthis manner, the stabilizer is capable of limiting the extent of pigmentparticle or molecular self-assembly so as to produce predominantlynanoscale pigment particles, be they primary nanoparticles(crystallites) which are typically less than 50 nm, or tightly-boundnanoscale aggregrates that are typically less than 100-150 nm. Inaddition to the functional moiety having high pigment affinity (referredto hereafter as “pigment-affinic” functional moiety), these stabilizercompounds can also possess one or more hydrophobic groups, such as longalkyl hydrocarbon groups, or alkyl-aryl hydrocarbon groups, wherein thealkyl groups can be linear, cyclic or branched in structure and have atleast 6 or more carbons in total. The presence of the additionalhydrophobic groups in such stabilizers can serve several functions: (1)to provide a sterically bulky layer surrounding the pigment particle,thereby preventing or limiting the approach of other pigment particlesor molecules that results in uncontrolled aggregation, and ultimatelyparticle growth; and (2) to help compatibilize the pigment for betterdispersability in the targeted vehicle or matrix. Such compounds havingboth a pigment-affinic functional moiety that associates noncovalentlywith the pigment, as well as one or more sterically bulky hydrocarbongroups that provide a surface barrier to other pigment particles, areoften referred to as “steric stabilizers” and have been used in variousways to alter the surface characteristics of conventional pigments andother particles requiring stabilization (for example, latex particles inpaints, metal oxide nanoparticles in anti-scratch coatings, amongothers).

The functional group on the stabilizer has a structure that is bothcomplementary and has high affinity for non-covalent bonding associationwith the one or more functional moieties on the pigment. Suitablecomplementary functional groups on the stabilizer include the following:sulfonate/sulfonic acid, (thio)carboxylate/(thio)carboxylic acid,phosphonate/phosphonic acid, ammonium and substituted ammonium salts,phosphonium and substituted phosphonium salts, substituted carboniumsalts, substituted arylium salts, alkyl/aryl (thio)carboxylate esters,thiol esters, primary or secondary amides, primary or secondary amines,hydroxyl, ketone, aldehyde, oxime, hydroxylamino, enamines (or Schiffbase), porphyrins, (phthalo)cyanines, urethane or carbamate, substitutedureas, guanidines and guanidinium salts, pyridine and pyridinium salts,imidazolium and (benz)imidazolium salts, (benz)imidazolones, pyrrolo,pyrimidine and pyrimidinium salts, pyridinone, piperidine andpiperidinium salts, piperazine and piperazinium salts, triazolo,tetraazolo, oxazole, oxazolines and oxazolinium salts, indoles,indenones, and the like.

The stabilizer compound should have a hydrocarbon moiety that providessufficient steric bulk to enable the function of the stabilizer toregulate pigment particle size. The hydrocarbon moiety in embodiments ispredominantly aliphatic, but in other embodiments can also incorporatearomatic groups, and generally contains at least 6 carbon atoms, such asat least 12 carbons or at least 16 carbons, and not more than about 100carbons, but the actual number of carbons can be outside of theseranges. The hydrocarbon moiety can be either linear, cyclic or branched,and in embodiments is desirably branched, and may or may not containcyclic moieties such as cycloalkyl rings or aromatic rings. Thealiphatic branches are long with at least 2 carbons in each branch, suchas at least 6 carbons in each branch, and not more than about 100carbons.

It is understood that the term “steric bulk” is a relative term, basedon comparison with the size of the pigment or pigment precursor to whichit becomes non-covalently associated. In embodiments, the phrase “stericbulk” refers to the situation when the hydrocarbon moiety of thestabilizer compound that is coordinated to the pigment/precursorsurface, occupies a 3-dimensional spatial volume that effectivelyprevents the approach or association of other chemical entities (e.g.colorant molecules, primary pigment particles or small pigmentaggregate) toward the pigment/precursor surface. Thus, the stabilizershould have its hydrocarbon moiety large enough so that as severalstabilizer molecules become non-covalently associated with the chemicalentity (pigment or precursor), the stabilizer molecules act as surfacebarrier agents for the primary pigment particles and effectivelyencapsulates them, and thereby limits the growth of the pigmentparticles and affording only nanoparticles of the pigment. For example,for the pigment precursor Lithol Rubine and for the organic pigmentPigment Red 57:1, the following illustrative groups on a stabilizer areconsidered to have adequate “steric bulk” so as to enable the stabilizerto limit the extent of pigment self-assembly or aggregation and mainlyproduce pigment nano-sized particles:

Classes of suitable stabilizer compounds include the following: themono- and di-carboxylic acids, mono- and di-esters, and mono- anddi-primary amide derivatives of long-chain, branched, or cyclic alkanes,alkenes, and alkylarenes; the mono- and di-carboxylic acids, mono- anddi-esters, and mono- and di-primary amide derivatives of heterocycliccompounds such as pyridine, piperidine, piperazine, morpholine andpyrroles; monosubstituted pyridine, piperazine, piperidine, morpholine,pyrrole, imidazole, thiazole and their cationic salts, wherein thesubstituent is a long-chain or branched aliphatic hydrocarbon;poly(vinyl pyrrolidone) and copolymers of poly(vinyl pyrrolidone) withα-olefins or other ethylenically unsaturated monomer compounds, such asfor example poly(vinyl pyrrolidone-graft-1-hexadecane) and poly(vinylpyrrolidone-co-eicosene) and the like; poly(vinyl imidazole) andcopolymers of poly(vinyl imidazole) with α-olefins or otherethylenically unsaturated monomer compounds; poly(vinyl pyridine) andcopolymers of poly(vinyl pyridine) with α-olefins or styrene, or otherethylenically unsaturated monomer compounds; long-chain or branchedaliphatic primary amides and amidines, including Guerbet-type primaryamides and amidines; semicarbazides and hydrazones of long-chainaliphatic and/or branched aldehydes and ketones; mono-substituted ureasand N-alkyl-N-methyl ureas, wherein the substituent is a long-chain orbranched aliphatic hydrocarbon; mono-substituted monosubstitutedguanidines and guanidinium salts, wherein the substituent is along-chain or branched aliphatic hydrocarbon; mono- and di-substitutedsuccinimides, such as 2-alkyl- and 2,3-dialkyl-succinimides, and mono-and di-substituted succinic acids or their esters, wherein one or morealkyl substituent is comprised of a long-chain or branched aliphatichydrocarbon having between 6 and 50 carbon atoms; mixtures thereof; andthe like.

Representative examples of stabilizer compounds that have both thefunctional group that non-covalently associates with the pigment and thesterically bulky hydrocarbon moiety, include (but are not limited to)the following compounds:

wherein m and n denotes the number of repeated methylene units, andwhere m can range between 1 and 50, and n can range between 1 and 5,however the values can also be outside these ranges. Additional examplesof suitable stabilizer compounds include (but are not limited to) thefollowing compounds:

In additional embodiments, other stabilizer compounds having differentstructures than those described previously may be used in addition tosterically bulky stabilizer compounds, to function as surface activeagents (or surfactants) that either prevent or limit the degree ofpigment particle aggregation. Representative examples of such surfaceactive agents include, but are not limited to, rosin natural productssuch as abietic acid, dehydroabietic acid, pimaric acid, rosin soaps(such as the sodium salt of the rosin acids), hydrogenated derivativesof rosins and their alkyl ester derivatives made from glycerol orpentaerythritol or other such hydrocarbon alcohols, acrylic-basedpolymers such as poly(acrylic acid), poly(methyl methacrylate),styrene-based copolymers such as poly(styrene sodio-sulfonate) andpoly(styrene)-co-poly(alkyl (meth)acrylate), copolymers of α-olefinssuch as 1-hexadecene, 1-octadecene, 1-eicosene, 1-triacontene and thelike, copolymers of 4-vinyl pyridine, vinyl imidazole, and vinylpyrrolidinone, polyamides, polyesters, polyester-amides and copolymersthereof, oligomers of amides, esters, and lactones, copolymers ofacetals and acetates, such as the copolymer poly(vinylbutyral)-co-(vinylalcohol)-co-(vinyl acetate).

The types of non-covalent chemical bonding that can occur between thefunctional moiety of the precursor/pigment and the complementaryfunctional group of the stabilizer are, for example, van der Waals'forces, ionic or coordination bonding, hydrogen bonding, and/or aromaticpi-stacking bonding. In embodiments, the non-covalent bonding ispredominately ionic bonding, but can include hydrogen bonding andaromatic pi-stacking bonding as additional or alternative types ofnon-covalent bonding between the functional moieties of the stabilizercompounds and the precursor/pigment.

The “average” pigment particle size, which is typically represented asd₅₀, is defined as the median particle size value at the 50th percentileof the particle size distribution, wherein 50% of the particles in thedistribution are greater than the d₅₀ particle size value and the other50% of the particles in the distribution are less than the d₅₀ value.Average particle size can be measured by methods that use lightscattering technology to infer particle size, such as Dynamic LightScattering. The term “particle diameter” as used herein refers to thelength of the pigment particle at the longest dimension (in the case ofacicular shaped particles) as derived from images of the particlesgenerated by Transmission Electron Microscopy (TEM). The term“nano-sized”, “nanoscale”, “nanoscopic”, or “nano-sized pigmentparticles” refers to for instance, an average particle size, d₅₀, or anaverage particle diameter of less than about 150 nm, such as of about 1nm to about 100 nm, or about 10 nm to about 150 nm, or about 50 to about80 or to about 100 nm. Geometric standard deviation is a dimensionlessnumber that typically estimates a population's dispersion of a givenattribute (for instance, particle size) about the median value of thepopulation and is derived from the exponentiated value of the standarddeviation of the log-transformed values. If the geometric mean (ormedian) of a set of numbers {A₁, A₂, . . . , A_(n)} is denoted as μ_(g),then the geometric standard deviation is calculated as:

$\sigma_{g} = {\exp\sqrt{\frac{\sum\limits_{i = 1}^{n}\;\left( {{\ln\mspace{14mu} A_{i}} - {\ln\mspace{14mu}\mu_{g}}} \right)^{2}}{n}}}$

In general, the methods of making nanoscale particles of azo or lakedazo pigments such as those listed in Table 7, and similarly othernanoscale particles of azo type dyes such as disazo and disazo lakedpigments, is a process that involves one or more reaction steps, and inembodiments, at least one of these reaction steps is performed within amicroreactor or micromixer. A diazotization reaction is a key reactionstep for synthesis of the monoazo and laked monoazo pigments, whereby asuitably substituted aniline precursor (denoted as diazo component DC)such as those listed in Table 1, and Formulas (2) and (3), is eitherdirectly or indirectly converted first to a diazonium salt usingstandard procedures, such as that which includes treatment with aneffective diazotizing agent such as nitrous acid HNO₂ (which isgenerated in situ by mixing sodium nitrite with dilute protic acidsolution such as hydrochloric acid), or nitrosyl sulfuric acid (NSA),which is commercially available or can be prepared by mixing sodiumnitrite in concentrated sulfuric acid. Initially, it may be necessary tofirst dissolve the precursor substituted aniline in alkaline solution(such as aqueous potassium hydroxide solution, or amimonia water)followed by treatment with the diazotizing agent and acid solution, soas to generate the diazonium salt. The diazotization procedure istypically carried out at cold temperatures so as to keep the diazoniumsalt stable, and the resulting reaction mixture will comprise mainly thediazonium salt either dissolved or suspended as a precipitate in acidicmedium. For laked pigments such as laked monoazo pigments, if desiredand effective, an aqueous solution of the metal salt (M^(n+)) can beoptionally added that will define the specific composition of thedesired monoazo laked pigment product, such as those listed in Table 7.

A second solution or suspension is prepared by dissolving or suspendingthe nucleophilic coupling component (denoted as CC, such as those shownin Tables 2-6, and Formulas (4)-(8)) mainly into water, which mayoptionally contain another liquid such as an organic solvent (forexample, iso-propanol, tetrahydrofuran, methanol, or other). Acids orbases can be used to render the coupling component into solution or afine suspension which can improve reactivity with the diazonium saltsolution, and additionally any buffers or surface active agentsincluding the sterically bulky stabilizer compounds such as thosedescribed previously, may be added to the second solution of couplingcomponent.

The first diazonium salt solution or suspension is combined in anysuitable manner with the second coupling component solution orsuspension, to produce a colored solid that is typically obtained assuspended particles in the aqueous medium. This reaction step may becarried out, using appropriate processing conditions, in either batchmode or by continuous processing, including the use of a microreactor ormicromixer unit. Both azo and laked azo pigments are generated in thismanner. For preparation of laked pigments, an alternative series ofreaction steps can be used that begins with combining the firstdiazonium salt solution, which is devoid of any metal cation salt, withthe second coupling component solution, in either a batch process or acontinuous (microreactor) process, to prepare a water-soluble dye thatis a synthetic precursor to the actual insoluble pigment. Thewater-soluble dye precursor is then rendered into homogeneous solutionor a fine suspension by treatment with either acids or bases, and theresultant homogeneous solution of the dye is then further reacted with asolution of the appropriate metal cation salt, in either a batch processor a continuous (microreactor or micromixer) process, to precipitate acolored pigment product as fine nanoparticles. There are severalchemical as well as physical processing factors that can affect thefinal particle size and distribution of the pigment nanoparticles,including stoichiometries of the DC and CC starting reactants,(optional) metal salt, surface active agents, and stabilizer compounds,the concentrations of chemical species in the liquid medium, pH ofliquid medium, temperature, addition rate, order of addition, agitationrate, post-reaction treatments such as heating, isolation and washing ofparticles, and drying conditions. If effective and desired, a variety offinishing steps may be performed to obtain the desired properties of thenanoscale pigment particles, which include careful heating for narrowingthe distribution of particle sizes and/or shapes, or optional surfacetreatment with resinous compounds to improve the pigment dispersionproperties.

The microreactor process uses pumps to feed the reactant solutions intothe microreactor where they are well mixed and react to form the productpigment suspension. Tubing or other connections can be used tointerconnect the feed tanks to the pumps and these to the microreactor.An outlet tubing line can then be connected to a product receiving vialor other product container. For the purpose of this disclosure, amicroreactor is defined as any mechanical assembly having a channeldimension, through which a fluid flows, of less than 1 mm. Specifically,the microreactor used in embodiments is a commercial CYTOS™ Lab System,manufactured by CPC Technologies. The microreactor used in embodimentsis defined as the combination of the CYTOSO Lab System Microreactor unitand a number of CYTOS™ Lab System Residence Time units. For each exampleprovided, the configuration of the microreactor will be defined (i.e.microreactor unit+X number of retention time units). The CYTOS™ LabSystem Microreactor unit has a total process fluid volume of 1.8 mLwhile the residence time units each hold a process fluid volume of 15mL. Each unit consists of a series of plates laminated together to formfluid flow channels for the process and heat exchange streams. Thisconfiguration thus allows the reaction temperature to be controlled byflowing independently a heat exchange medium through the microreactorthat does not contact directly the process streams. Of course, it isunderstood and will be apparent that the process described herein is notlimited to this particular microreactor configuration. Rather, theprocess is applicable to any device that leads to the conveying offluids in channels less than 1 mm in laminar flow for the purpose ofsynthesizing and precipitating nanoscale particles of azo pigments,which may involve a laking reaction step, brought about by thecontacting or combining of two or more streams over a specified durationof time in a confined flow environment. For example the microreactor canbe replaced by a micromixer (such as for example the Caterpillar mixermodel CPMM-R300 purchased from Institut für Mikrotechnik Mainz GmbH) toprovide good mixing of the two fluids and in addition a downstream tubeof the appropriate size to achieve a desired residence time.

An exemplary diagram of the process of the disclosure is shown in FIG.1.

The process in an exemplary embodiment of the disclosure is operatedwith two feed streams to the microreactor and one product stream,although other configurations may be used. One feed stream contains theprecursor dye and a second feed stream contains a metal-salt solution(in this example CaCl₂). The sterically bulky stabilizer compound andsurfactant may be added in the precursor dye stream or as separatestreams. However, the sterically bulky stabilizer compound andsurfactant should not be included with the metal-salt solution, as thiswould lead to premature precipitation of these materials. In thisembodiment, the precursor dye stream contains both the surfactant andsterically bulky stabilizer compound. The streams are fed into themicroreactor at flowrates ranging between about 0.1 and about 100 mL/mineach, such as about 10 mL/min each. Pumps convey these fluids from feedtanks to the microreactor. The microreactor is preheated or cooled tooperate in a specific temperature window, which is between about 0° C.and about 25° C.

As each stream enters the microreactor it is subdivided and thencombined in an intermeshing fashion with the other stream. Thisintermeshing of the fluid lamellae, of micron scale thickness, leads torapid mixing of the two or more fluid streams. As the fluids are mixedand progress down the flow channel, laking (ion-exchange reactionleading to precipitation of pigment) takes place. On account of thepresence of the sterically bulky stabilizer compound and surfactant,which limits the growth of the pigment to the nanometer scale, thepigment product solution continues to flow through the microreactor(rather than blocking the small channels) even under laminar flowconditions (that is, a Reynold's number less than about 2300). However,in embodiments, the flow in the microreactor or micromixer can belaminar or turbulent, and can have a Reynold's number of, for example,from about 10 to about 10,000. The total residence time in themicroreactor is important to the completion of the reaction asinsufficient reaction time in the microreactor can lead to incompleteconversion and the formation of larger pigments in the productcollection tank. Residence times in the range of about 0.04 seconds toabout 2.1 minutes have been used, and it has been found that the bestresults, smallest nanoparticles formed, are achieved with residencetimes in the order of a few minutes. However, there is no generallimitation on the residence time, and residence times of up to about 1hour, up to about 2 hours, or even up to about a day or more, could beused. This residence time was unexpected, because it was previouslybelieved that the reaction in batch was instantaneous.

The reaction is conducted in a microreactor, optionally with one or moreresidence time plates. Any suitable microreactor and residence timeplates may be used, and are not limited to the described commercialCytos reactors. When used, the residence time plates generally containextended channels with good heat transfer but no active mixing elements.

As needed, the microreactor can be heated or cooled by pumping heatingor cooling fluid from an external bath through the heating side of themicroreactor. In embodiments, the temperature of the microreactor canlie held at any desired value within wide limits. Likewise, if desired,the feed streams containing the reactants and other inputs can also befed to the microreactor at the same or different temperatures.Additionally, the reaction can be carried out in the microreactor atpressures between atmospheric pressure and about 100 bar, for examplebetween atmospheric pressure and about 25 bar, although pressures aretypically between atmospheric pressure and about 2 bar.

The preparation of mixtures of input materials to form streams ofmaterials may be carried out in advance in micromixers or in upstreammixing zones, as appropriate. The input materials can then be introducedinto a microreactor individually or as mixtures.

It is surprising and was unforeseeable that the preparation of nanoscalepigment particle compositions in a microreactor would be possible inthis technically elegant manner. In particular, it was surprising andwas unforeseeable that such controlled size nanoparticles could beformed in a microreactor, without the particles growing to such a sizethat would clog the microreactor channels.

After the pigment synthesis is completed, if effective and desired, avariety of finishing steps may be performed to obtain the desiredproperties of the pigment particles, which include careful heating fornarrowing the distribution of particle sizes and/or shapes, or optionalsurface treatment with resinous compounds to improve the pigmentdispersion properties. The isolation of the pigment product can be donein any suitable manner, including separation, extraction, filtration,and/or purification processes or recrystallization is needed, which canbe conducted as desired to a desired purity level. For example, thedesired nanoparticle product can be subjected to conventional washingsteps, can be separated, treated with known absorbents (such as silica,alumina, and clays, if necessary) and the like. The final product canalso be dried, for example, by air drying, vacuum drying, or the like.All of these procedures are conventional and will be apparent to thoseskilled in the art.

In a specific embodiment, the preparation of ultrafine and nanosizedparticles of the monoazo laked Pigment Red 57:1 was only enabled by theadditional use of a sterically bulky stabilizer compound having afunctional group that could non-covalently bond to the complementaryfunctional moiety of the pigment as well as branched aliphaticfunctional groups that could provide steric bulk to the pigment particlesurface. In embodiments, particularly suitable sterically bulkystabilizer compounds are branched hydrocarbons with either carboxylateor sulfonate functional groups, compounds such asdi[2-ethylhexyl]-3-sulfosuccinate sodium or sodium 2-hexyldecanoate, andthe like. The stabilizer compound is introduced as a solution orsuspension in a liquid that is predominantly aqueous but may optionallycontain a water-miscible organic solvent such as THF, iso-propanol, NMP,Dowanol and the like, to aid dissolution of the stabilizer compound, inan amount relative to colorant moles ranging from about 5 mole-percentto about 100 mole-percent, such as from about 20 mole-percent to about80 mole-percent, or from about 30 mole-percent to about 70 mole-percent,but the concentrations used can also be outside these ranges and inlarge excess relative to moles of colorant.

Lastly, the metal cation salt is added to transform the pigmentprecursor (Lithol Rubine-potassium salt in embodiments) into the desiredmonoazo laked pigment (Pigment Red 57:1 in embodiments), precipitated asnano-sized pigment particles. In a batch process the aqueous solution ofmetal salt (calcium chloride in embodiments) with concentration ranginganywhere from 0.1 mol/L to about 2 mol/L, is slowly added dropwise innearly stoichiometric quantities such as amounts ranging from 1.0 molarequivalents relative to about 2.0 molar equivalents, or from 1.1 toabout 1.5 molar equivalents, or from 1.2 to about 1.4 molar equivalentsrelative to moles of colorant, however the amounts used can also beoutside of these ranges and in large excess. In a continuousmicroreactor process the addition of the metal-salt solution can be donequickly on account of the efficient and fast mixing afforded by themicroreactor.

The type of metal salt can have an impact on the extent of formingnano-sized pigment particles of monoazo laked pigments, in particularthe type of ligand that is coordinated to the metal cation and therelative ease with which it is displaced by a competing ligand fromeither the pigment functional moiety or the complementary functionalmoiety of the stabilizer compound, or both. In embodiments for monoazolaked Pigment Red 57:1, the nano-sized particles are formed usingcalcium (II) salts with ligands such as chloride, sulfate, acetate, andhydroxide; however a particularly desirable metal salt is calciumchloride for fastest reactivity.

Temperature during the pigment precipitation step using the metal saltsolution is also important. In embodiments, lower temperatures aredesired, such as from about 0° C. to about 50° C., or from about 0° C.to about 25° C., but the temperature can also be outside of theseranges.

Characterization of the chemical composition of washed and driednano-sized pigment particles are performed by NMR spectroscopy andelemental analysis. In a specific embodiment, the composition of themonoazo laked pigment Red 57:1 indicated that the nano-sized particlesprepared by the methods described above, particularly when usingdi[2-ethylhexyl]-3-sulfosuccinate sodium as the sterically bulkystabilizer, retained at least 80% of the sterically bulky stabilizerthat was loaded into the process of making the nanoparticles, even aftercopious washing with deionized water to remove excess salts. Solid state¹H- and ¹³C-NMR spectroscopic analyses indicated that the stericstabilizer compound was associated non-covalently with the pigment as acalcium salt, and the chemical structure of the pigment adopted thehydrazone tautomer form, as shown in Figure below.

Pigment particles of azo laked pigments such as PR 57:1 that havesmaller particle sizes could also be prepared by the above method in theabsence of using sterically bulky stabilizers and with the use ofsurface active agents alone (for example, only rosin-type surfaceagents), depending on the concentrations and process conditionsemployed, but the pigment product did not predominantly exhibitnano-sized particles nor did the particles exhibit regular morphologies.In the absence of using the sterically bulky stabilizer compound, themethods described above typically produced rod-like particle aggregates,ranging in average particle diameter from 200-700 nm and with wideparticle distribution, and such particles were difficult to disperseinto a polymer coating matrix and generally gave poor coloristicproperties. In embodiments, the combined use of a suitable stericallybulky stabilizer compound with a minor amount of suitable surface activeagent such as derivatives of rosin-type surfactants, using either of thesynthesis methods described previously would afford the smallest finepigment particles having nanometer-scale diameters, more narrow particlesize distribution, and low aspect ratio. Various combinations of thesecompounds, in addition to variations with process parameters such asstoichiometry of reactants, concentration, addition rate, temperature,agitation rate, reaction time, and post-reaction product recoveryprocesses, enables the formation of pigment particles with tunableaverage particle size (d₅₀) from nanoscale sizes (about 1 to about 100nm) to mesoscale sizes (about 100 to about 500 nm) or larger. Thedispersion ability and coloristic properties (L*, a*, b*, chroma, hueangle, light scatter index) of the pigment particles in a thin polymerbinder coating were directly correlated to the average pigment particlesize, which in turn was impacted by both the structural composition andamount of sterically bulky stabilizer compound (relative to molar amountof pigment) that was employed within the synthesis process.

The advantages of this process include the ability to tune particle sizeand composition for the intended end-use application of the monoazopigments, such as toners and inks and coatings, which includephase-change, gel-based and radiation-curable inks, solid and non-polarliquid inks, solvent-based inks and aqueous inks and ink dispersions.For the end-use application in piezoelectric inkjet printing, nanoscaleparticles are advantageous to ensure reliable inkjet printing andprevent blockage of jets due to pigment particle agglomeration. Inaddition, nanoscale pigment particles are advantageous for offeringenhanced color properties in printed images, since in embodiments thecolor properties of nanoscale particles of monoazo laked pigment Red57:1 were tunable with particle size, whereby as average particle sizewas decreased to nanometer scale, the hue angles were shifted fromyellowish-red hues to bluish-red hues by an amount ranging from about 5to about 35° in the color gamut space.

In embodiments, the nanoscale pigment particles that were obtained formonoazo laked pigments can range in the average particle size, d₅₀, orin the average particle diameter, from about 10 nm to about 250 nm, suchas from about 25 nm to about 175 nm, or from about 50 nm to about 150nm, as measured by either dynamic light scattering method or from TEMimages. In embodiments, the reaction in a microreactor advantageouslyprovides monoazo laked pigment particles having an average particle sizeor diameter of from about 10 m to about 150 run, such as about 50 toabout 100 nm. In embodiments, the particle size distributions can rangesuch that the geometric standard deviation (denoted as GSD) can rangefrom about 1.1 to about 1.9, or from about 1.2 to about 1.7, as measuredby dynamic light scattering method. The shape of the nanoscale pigmentparticles can be one or more of several morphologies, including rods,platelets, needles, prisms or nearly spherical, and the aspect ratio ofthe nanoscale pigment particles can range from 1:1 to about 10:1, suchas having aspect ratio between 1:1 and 5:1; however the actual metriccan lie outside of these ranges.

The color of the nanoscale pigment particles have the same general hueas is found with larger pigment particles. However, in embodiments, isdisclosed coloristic properties of thin coatings of the nanoscalepigment particles of red monoazo pigments dispersed in a polymer binder(such as of poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate)), thatexhibited a significant shift to lower hue angle and lower b* valuesthat revealed more bluish magenta hues, and having either no change or asmall enhancement of a* value. In embodiments, the hue angles of thecoatings dispersed with the nanoscale particles of monoazo laked pigmentsuch as Pigment Red 57:1 measured in the range from about 345° to about5° on the 2-dimensional b* a* color gamut space, as compared with hueangles ranging from about 0° to about 20° for similarly prepared polymercoatings dispersed with conventional larger sized particles of PigmentRed 57:1. In embodiments is disclosed the coloristic properties (hueangle, a*, b*, and NLSI as measure of specular reflectivity) ofnanoscale pigment particles, particularly of monoazo laked red pigment,that are directly correlated and tunable with the average pigmentparticle size, measured by either Dynamic Light Scattering or electronmicroscopy imaging techniques, as well as pigment composition with thenon-covalently associated stabilizer, the latter which enables thecontrol of particle size during pigment synthesis, and also enablesenhanced dispersability within certain polymer binders for coating orother applications.

Additionally, the specular reflectivity of the coatings of the nanoscalemonoazo laked red pigment was significantly enhanced from coatingsproduced with conventional larger sized pigment particles, which is anindicator of having very small particles being well-dispersed within thecoating. Specular reflectivity was quantified as the degree of lightscattering for the pigmented coating, a property that is dependent onthe size and shape distributions of the pigment particles and theirrelative dispersability within the coating binder. The Normalized LightScatter Index (NLSI) was quantified by measuring the spectral absorbanceof the coating in a region where there is no absorbance from thechromogen of the pigment, but only absorbance due to light scatteredfrom large aggregates and/or agglomerated pigment particles dispersed inthe coating binder. The light scattering absorbance data is thennormalized to a lambda-max optical density of 1.5, resulting in the NLSIvalue, in order to directly compare the light scattering indices ofseveral pigmented coatings. The lower is the NLSI value, the smaller isthe pigment particle size within the dispersed coating matrix. Inembodiments, the NLSI values obtained for the nanoscale monoazo lakedred pigments can range from about 0.1 to about 3.0, such as from about0.1 to about 1.0, as compared to the NLSI values observed with similarlyprepared coatings containing larger sized monoazo laked red pigmentsthat range anywhere from about 3.0 to about 75 (a very poorly dispersedcoating).

The formed nanoscale pigment particle compositions can be used, forexample, as coloring agents in a variety of compositions, such as inliquid (aqueous or non-aqueous) ink vehicles, including inks used inconventional pens, markers, and the like, liquid ink jet inkcompositions, solid or phase change ink compositions, and the like. Forexample, the colored nanoparticles can be formulated into a variety ofink vehicles, including “low energy” solid or gel-type inks with melttemperatures of about 60 to about 130° C., solvent-based liquid inks orradiation-curable such as UV-curable liquid inks comprised ofalkyloxylated monomers, and even aqueous inks.

Examples are set forth herein below and are illustrative of differentcompositions and conditions that can be utilized in practicing thedisclosure. All proportions are by weight unless otherwise indicated. Itwill be apparent, however, that the disclosure can be practiced withmany types of compositions and can have many different uses inaccordance with the disclosure above and as pointed out hereinafter.

EXAMPLES Comparative Example 1 Synthesis of Pigment Red 57:1 by aTwo-Step Procedure

Step 1: Synthesis of Lithol Rubine Potassium Salt, a Dye Precursor forPigment Red 57:1

Diazotization: Into a 500 mL round bottom flask equipped with amechanical stirrer, thermometer, and addition funnel was dissolved2-amino-5-methylbenzenesulfonic acid (8.82 g, 47.1 mmol) into 0.5M KOHaqueous solution (97.0 mL). A medium brown solution was formed, whichwas cooled to between 0° C. and 2° C. A 20 wt % aqueous solution ofsodium nitrite (NaNO₂; 3.28 g, 47.6 mmol dissolved into 25 mL water) wasadded slowly to the first solution, maintaining the temperature below 3°C., which resulted in a red-brown solution. Concentrated HCl (10 M,14.15 mL, 141.5 mmol) was then added dropwise while maintaining theinternal temperature below 2° C. The mixture formed a light brownsuspension. After complete addition of conc. HCl, the suspension wasstirred an additional 30 min.

Coupling: In a separate 2-L resin kettle was dissolved3-hydroxy-2-naphthoic acid (8.86 g, 47.1 mmol) into an aqueous solutionof KOH (8.72 g, 155.4 mmol) in water (95 mL). Additional water was added(250 mL), and the light-brown solution was then cooled to about 15° C.while stirring vigorously. The cold suspension of the diazonium saltsolution from Step A was then transferred slowly into the couplingsolution of Step B, while mixing vigorously. The color changed from darkred solution, to ultimately a yellowish-red (orange) slurry ofprecipitated colorant. The mixture was stirred for another 2 hours atroom temp, then vacuum-filtered and diluted with water (500 mL) toprovide an orange aqueous slurry of Lithol Rubine Dye Potassium salt,with approximate solids content of about 1.6%-wt.

Step 2: Laking Process Step.

Into a 500 mL round bottom flask equipped with mechanical stirrer andcondenser was charged 125.8 g of aqueous slurry of Lithol Rubine DyePotassium salt from Example 1 (1.6% wt solids content, 2.0 grams ofcolorant, 4.33 mmol). The pH of the slurry was adjusted to above 9.0 bythe addition of 0.5 M KOH solution. A solution of calcium chloridedehydrate (0.5 M solution in water, 13 mL) was added dropwise to theslurry while stirring vigorously. A red precipitate formed immediately,and after addition was completed, the slurry was stirred for another 2hours. The slurry was then heated to about 75° C. for 20 min, thencooled to room temp. The slurry was vacuum-filtered using a 0.8 μm Nylonmembrane cloth, then reslurried twice with water (150 mL portions). ThepH and conductivity of the filtrates after each filtration were measuredand recorded, with the second (final) wash filtrate having pH of 6.17and conductivity of about 13.5 μS/cm. The red pigment filtercake wasreslurried into about 200 mL of water and freeze-dried, to afford a redcolored powder (1.92 grams).

Comparative Example 2 Preparation of NanoPigment Red 57:1 Using StericStabilizer, in a Batch Process

Laking step in batch process: Into a 500 mL round bottom flask equippedwith mechanical stirrer and condenser was charged 125.8 g of aqueousslurry of Lithol Rubine Potassium salt from Comparative Example 1 (1.6%wt solids content, 2.0 grams of colorant, 4.33 mmol). The pH of theslurry was adjusted to above 9.0 by the addition of 0.5 M KOH solution.An aqueous solution of a rosin soap (5 wt % Dresinate X, 4.0 mL) wasadded while stirring, followed by a prepared solution containing sodiumdioctyl sulfosuccinate (0.96 g, 2.17 mmol) dissolved in 90:10 water/THFmixture (100 mL), added while stirring the slurry vigorously. An aqueoussolution of calcium chloride dihydrate (0.5 M solution, 13 mL, 6.50mmol) was added dropwise to the slurry while stirring vigorously. A redprecipitate formed immediately, and after addition was completed, theslurry was stirred for an additional 1.5 hours. The red slurry was thenheated to about 75° C. for 20 min, then cooled to room temp. The slurrywas vacuum-filtered through a 0.8 μm Nylon membrane cloth, thenreslurried twice with water. The pH and conductivity of the filtratesafter each filtration were measured and recorded, with the second(final) wash filtrate having pH of 7.4 and conductivity of about 110μS/cm. The red pigment filtercake was reslurried into water (200 mL) andfreeze-dried to afford a brown-red colored powder (2.65 grams).

Example 1 Preparation of NanoPigment Red 57:1 Using Steric Stabilizer ina Microreactor Process (Microreactor Used is Cytos Lab System Purchasedfrom Micro-Reactor Systems Provider, Inc,) with the FollowingConfiguration: Microreactor Unit+no Residence Time Units, Operating at25° C. and 20 mL/min

Preparation of feed stream 1: In an Erlenmeyer flask (250 mL) LitholRubine Potassium Salt dye precursor solids (1.0 g solids) were dilutedto 1.0 wt % slurry (100 g). To this slurry was added about 2 mL of a 5wt % solution of Dresinate-X in 80:20 H₂O/THF mixture and 10.8 mL from0.1M solution of sodium dioctyl sulfosuccinate dissolved in 90:10H₂O/THF). The pH of this solution was then adjusted to above 10 with1.0M KOH solution, which gave a dark red, homogeneous solution.

Preparation of feed stream 2: In an Erlenmeyer flask was prepared a0.02M aqueous solution of CaCl₂.

Laking in Microreactor: Feed stream 1 containing sodium dioctylsulfosuccinate, Dresinate-X, potassium hydroxide solution and LitholRubine dye precursor and Feed steam 2 containing 0.02 M CaCl₂ solutionwere fed at flowrates of 10 mL/min respectively using Waters HPLC pumpsinto the microreactor preheated at 25° C. (microreactor plate+noresidence time units, residence time of 5 seconds, Reynolds number of520 indicating laminar flow). Once the process reached a steady state,the product stream was collected on a 0.8 μm membrane filter andfiltered continuously.

Sample Preparation for TEM analysis: Approximately 2 mL of n-BuOH wasadded to re-disperse the pigment filtercake from the membrane. Allsamples were sonicated for about 5 minutes to promote dispersion. Arepresentative droplet from each sample was pipetted onto acarbon-coated copper grid and allowed to dry before being examined. Thetwo samples were examined using a Philips (now FEI) CM20 transmissionelectron microscope operated at 80 KV.

Example 2 Preparation of NanoPigment Red 57:1 Using Steric Stabilizer ina Microreactor Process (Microreactor Used is Cytos Lab System Purchasedfrom Micro-Reactor Systems Provider, Inc,) with the FollowingConfiguration: Microreactor Unit+no Residence Time Units, Operating at0° C. and 20 mL/min

Preparations of feed streams 1 and 2, and work-up are as described inExample 1.

Laking in Microreactor: Feed stream 1 containing sodium dioctylsulfosuccinate, Dresinate-X, potassium hydroxide solution and LitholRubine dye precursor and Feed steam 2 containing 0.02 M CaCl₂ solutionwere fed at flowrates of 10 mL/min respectively using Waters HPLC pumpsinto the microreactor cooled to 0° C. (microreactor plate+no residencetime units, residence time of 5 seconds, Reynolds number of 520indicating laminar flow). Once the process reached a steady state, theproduct stream was collected on a 0.8 μm membrane filter and filteredcontinuously.

Example 3 Preparation of NanoPigment Red 57:1 using Steric Stabilizer ina Microreactor Process (Microreactor Used is Cytos Lab System Purchasedfrom Micro-Reactor Systems Provider, Inc,) with the FollowingConfiguration: Microreactor Unit+Two Residence Time Units, Operating at25° C. and 15 mL/min

Preparation of feed stream 1: In an Erlenmeyer flask (250 mL) LitholRubine Potassium Salt dye precursor solids (1.5 g solids, fromExample 1) were diluted to 1.0 wt % slurry (150.3 g). To this slurry wasadded about 3 mL of a 5 wt % solution of Dresinate-X in 80:20 H₂O/THFmixture and 16.3 mL from 0.1M solution of sodium dioctyl sulfosuccinatedissolved in 90:10 H₂O/THF). The pH of this solution was then adjustedto above 10 with 1.0M KOH solution which gave a dark red, homogeneoussolution.

Preparation of feed stream 2: In an Erlenmeyer flask was prepared anaqueous solution of 0.02 M CaCl₂.

Laking in Microreactor: Feed stream 1 containing sodium dioctylsulfosuccinate, Dresinate-X, potassium hydroxide solution and LitholRubine dye precursor and Feed steam 2 containing 0.02 M CaCl₂ solutionwere fed at flowrates of 7.5 mL/min respectively using Waters HPLC pumpsinto the microreactor set at 25° C. (microreactor plate+two residencetime units, residence time of 2.1 minutes, Reynolds number of 390indicating laminar flow). Once the process reached a steady state, theproduct stream was collected on a 0.8 μm membrane filter and filteredcontinuously.

Work-up for TEM analysis is as described in Example 2.

Example 4 Preparation of NanoPigment Red 57:1 Using Steric Stabilizer ina Microreactor Process (Microreactor Used is Cytos Lab System Purchasedfrom Micro-Reactor Systems Provider, Inc,) with the FollowingConfiguration: Microreactor Unit+Two Residence Time Units, Operating at0° C. and 20 mL/min

Preparation of feed stream 1: In an Erlenmeyer flask (250 mL) LitholRubine Potassium Salt dye precursor solids (1.4 g solids, fromComparative Example 1) were diluted to 1.0 wt % slurry (140 g). To thisslurry was added about 2.8 mL of a 5 wt % solution of Dresinate-X in80:20 H₂O/THF mixture and 16.3 mL from 0.1M solution of sodium dioctylsulfosuccinate dissolved in 90:10 H₂O/THF). The pH of this solution wasthen adjusted to above 10 with 1.0M KOH solution which gave a dark red,homogeneous solution.

Preparation of feed stream 2: In an Erlenmeyer flask was prepared anaqueous solution of 0.02 M CaCl₂.

Laking in Microreactor: Feed stream 1 containing sodium dioctylsulfosuccinate, Dresinate-X, potassium hydroxide solution and LitholRubine dye precursor and Feed steam 2 containing 0.02 M CaCl₂ solutionwere fed at flowrates of 10 mL/min respectively using Waters HPLC pumpsinto the microreactor cooled to 0° C. (microreactor plate+two residencetime units, residence time of 1.6 minutes, Reynolds number of 520indicating laminar flow). Once the process reached a steady state, theproduct stream was collected in a vessel under agitation. Aliquots wereobtained from the product stream and filtered on a 0.8 μm membranecontinuously for TEM analysis.

Work-up for TEM analysis is as described in Example 2.

Example 5 Preparation of NanoPigment Red 57:1 Using Steric Stabilizer ina Micromixer Process with the Following Configuration: Caterpillar MixerModel CPMM-R300 (Purchased from Institut für Mikrotechnik Mainz GmbH)with Internal Volume of 10 μl Operating at Room Temperature and 15mL/min.

Preparation of feed stream 1: In an Erlenmeyer flask (250 mL) LitholRubine Potassium Salt dye precursor solids (0.42 g solids, fromComparative Example 1) were diluted to 1.0 wt % slurry (42.0 g). To thisslurry was added about 0.85 mL of a 5 wt % solution of Dresinate-X in80:20 H₂O/THF mixture and 4.55 mL from 0.1M solution of sodium dioctylsulfosuccinate dissolved in 90:10 H₂O/THF). The pH of this solution wasthen adjusted to above 10 with 1.0M KOH solution which gave a dark red,homogeneous solution.

Preparation of feed stream 2: In an Erlenmeyer flask was prepared anaqueous solution of 0.02 M CaCl₂.

Laking in Micromixer: Feed stream 1 containing sodium dioctylsulfosuccinate, Dresinate-X, potassium hydroxide solution and LitholRubine dye precursor and Feed steam 2 containing 0.02 M CaCl₂ solutionwere fed at flowrates of 7.5 mL/min respectively using Waters HPLC pumpsinto the micromixer at room temperature (residence time of 40microseconds, Reynolds number of 840 indicating laminar flow). Once theprocess reached a steady state, the product stream was collected on a0.8 μm membrane filter and filtered continuously.

Work-up for TEM analysis is as described in Example 2.

Example 6 TEM Characterization of NanoPigments

Sample Preparation for TEM analysis: Approximately 2 mL of n-BuOH wasadded to re-disperse the pigment filtercake from the membrane. Allsamples were sonicated for about 5 minutes to promote dispersion. Arepresentative droplet from each sample was pipetted onto acarbon-coated copper grid and allowed to dry before being examined. Thetwo samples were examined using a Philips (now FEI) CM20 transmissionelectron microscope operated at 80 KV.

FIG. 2 a is a lab control sample of Pigment Red 57:1, (prepared as inComparative Example 1 above and which has larger-sized particles). FIG.2 b is an image of nanopigment particles of PR 57:1, prepared by amicroreactor process as in Example 4, with 1 microreactor unit and tworesidence time units at a flow rate of 20 mL/min and at temperature of0° C.

TEM particle size measurements show that the pigments produced in abatch two step process (Comparative Example 1) are rod like particleswith large aspect (length: width) ratios. Particle lengths range fromabout 150 nm to >800 nm with an average particle size in the range of300-500 nm; particle widths range from approximately 40 nm to 100 nmresulting in average aspect ratios of approximately 7:1, length towidth. Pigments produced by the microreactor using SurfactantStabalizers (Example 4) are of smaller size and range from approximately30 nm to 200 nm in length with an average particle size in the range of50 nm to 100 nm. Particles greater than 100 nm are multi-facetedindicating that they are aggregates of nanoscale particles. The averagewidth of the particles is in the range of 20 to 50 nm with mostparticles measuring approximately 30 nm. The aspect ratio of particlesin Example 4 is therefore 3:1, length to width.

Example 7a Preparation of Liquid Dispersions and Coatings

Into a 30 mL amber bottle was added 0.22 g of Permanent Rubine P-L7B 01,available from Clariant Corporation, 0.094 g polyvinylbutyral terpolymer(B30HH obtained from Hoescht), 7.13 g n-butyl acetate (glass-distilledgrade, obtained from Caledon Laboratories) and 70.0 g of ⅛″ stainlesssteel shot (Grade 25 440C obtained from Hoover Precision Products). Thebottle was transferred to a jar mill and allowed to gently mill for 4days at about 100 RPM. Two draw-down coatings were obtained from theresultant dispersion using an 8-path gap on clear Mylar® film such thatthe wet thicknesses for each coating comprised of PR 57:1 pigment samplewere 0.5 and 1 mil. The air-dried coatings on clear Mylar® film werethen dried in a horizontal forced-air oven at 100° C. for 20 minutes.

Example 7b Preparation of Liquid Dispersion and Coatings

A dispersion and subsequent coatings thereof were prepared in the samemanner as in example 7a except that the pigment used was fromComparative Example 1.

Example 7c Preparation of Liquid Dispersion and Coatings

A dispersion and subsequent coatings thereof were prepared in the samemanner as in example 7a except that the pigment used was fromComparative Example 2.

Example 7d Preparation of Liquid Dispersion and Coatings

A dispersion and subsequent coatings thereof were prepared in the samemanner as in example 7a except that the pigment used was from Example 3.

Example 7e Preparation of Liquid Dispersion and Coatings

A dispersion and subsequent coatings thereof were prepared in the samemanner as in example 7a except that the pigment used was from Example 4.

Example 8 Evaluation of Coatings prepared from Liquid PigmentDispersions

The coloristic properties of the Mylar® coatings prepared in Examples7a, 7b, 7c, 7d and 7e were determined using an X-RITE 938spectrodensitometer, D₅₀, 2° measurement mode. L* a* b* and opticaldensity (O.D.) values were obtained for each of the samples, and the L*a* b* were normalized to an optical density of 1.5, and then used tocalculate the hue angle and chroma (C*), as listed in Table 8. The CIEL*a*b* Delta E metric (AE*) was used to compare the differences in L* a*b* of coatings made in Examples 7a, 7b, 7d and 7e against the referenceL* a* b* of coatings made in Example 7c. The coloristic properties ofthe pigments from Examples 3 and 4 made by a microreactor process werevery similar to those measured for the nanopigment from ComparativeExample 2 made by a batch process. AE* was small with visuallyimperceptible differences among the samples made in Examples 3 and 4relative to Comparative Example 2. In summary, the nanopigment PR 57:1samples prepared by a microreactor had excellent dispersability andcoatability attributes in great similarity to the comparativenanopigment sample prepared by a two step batch process. In addition themicroreactor samples offer, in contrast to commercially availablepigments (7 a) and pigments produced without steric stabilizers (7 b),coatings with bluish hue angle shifts less than about 0 degrees,especially less than about 350 degrees.

TABLE 8 Coloristic properties normalized to O.D. = 1.5 Example ExampleExample Example Example Metric 7a 7b 7c 7d 7e L* 47.86 44.78 50.76 51.5652.25 a* 71.06 71.52 77.33 78.14 78.10 b* 8.73 34.78 −15.62 −14.83−14.53 Hue Angle 7.0 25.9 348.6 349.3 349.5 (°) C* 71.6 79.5 78.6 79.879.4 ΔE* 25.3 51.1 reference 1.4 2.0

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims.

1. A process for preparing nanoscale azo pigment particles, comprising: providing an organic pigment precursor that contains at least one functional moiety, providing a sterically bulky stabilizer compound that contains at least one functional group, and carrying out a chemical reaction to form a pigment composition in a microreactor or micromixer, whereby the functional moiety found on the pigment precursor is incorporated within the pigment and non-covalently associated with the functional group of the stabilizer, so as to allow formation of nanoscale-sized pigment particles.
 2. The process of claim 1, wherein the azo pigment particles are selected from the group consisting of monoazo pigment particles, monoazo laked pigment particles, disazo pigment particles, and disazo laked pigment particles.
 3. The process of claim 1, further comprising: forming a first solution or suspension comprising the organic pigment precursor; and forming a second solution or suspension comprising a metal salt; wherein said carrying out a chemical reaction comprises feeding the first solution or suspension and the second solution or suspension into the microreactor or micromixer.
 4. The process of claim 3, wherein the sterically bulky stabilizer compound is fed to the microreactor or micromixer as part of the first solution or suspension, or as a separate feed stream.
 5. The process of claim 1, further comprising: forming a first solution or suspension comprising the organic pigment precursor and substantially no metal cation salt; and forming a second solution or suspension comprising a metal salt; wherein said carrying out a chemical reaction comprises: feeding the first solution or suspension and the second solution or suspension into the microreactor or micromixer; combining the first solution or suspension and the second solution or suspension to form a water-soluble dye; treating the water-soluble dye with an acid or a base to form a homogeneous solution or a fine suspension; and reacting the water-soluble dye with a solution of a metal cation salt to precipitate the nanoscale-sized pigment particles.
 6. The process of claim 1, wherein reactants are fed to the microreactor or micromixer at flowrates of between about 0.1 and about 100 mL/min each.
 7. The process of claim 1, wherein a Reynold's number of fluid flowing through the microreactor or micromixer is from about 10 to about 10,000.
 8. The process of claim 1, wherein the microreactor or micromixer is maintained at a temperature of from about 0° C. and about 25° C. during the chemical reaction.
 9. The process of claim 1, wherein flow of the organic pigment precursor and the sterically bulky stabilizer compound in the microreactor or micromixer is under laminar flow conditions.
 10. The process of claim 1, wherein flow of the organic pigment precursor and the sterically bulky stabilizer compound in the microreactor or micromixer is under turbulent flow conditions.
 11. The process of claim 1, wherein a residence time in the microreactor or micromixer is from about 0.04 seconds to about 1 hour.
 12. The process of claim 1, wherein the microreactor or micromixer comprises one or more residence time plates or units.
 13. The process of claim 1, wherein the nanoscale-sized pigment particles have an average particle diameter as derived from transmission electron microscopy imaging, of less than about 150 nm.
 14. The process of claim 1, wherein the at least one functional moiety of the organic pigment precursor is selected from the group consisting of sulfonate/sulfonic acid, (thio)carboxylate/(thio)carboxylic acid, phosphonate/phosphonic acid, ammonium and substituted ammonium salts, phosphonium and substituted phosphonium salts, substituted carbonium salts, substituted arylium salts, alkyl/aryl (thio)carboxylate esters, thiol esters, primary and secondary amides, primary and secondary amines, hydroxyl, ketone, aldehyde, oxime, hydroxylamino, enamines, porphyrins, (phthalo)cyanines, urethane, carbamate, substituted ureas, guanidines and guanidinium salts, pyridine and pyridinium salts, imidazolium and (benz)imidazolium salts, (benz)imidazolones, pyrrolo, pyrimidine and pyrimidinium salts, pyridinone, piperidine and piperidinium salts, piperazine and piperazinium salts, triazolo, tetraazolo, oxazole, oxazolines and oxazolinium salts, indoles, indenones, and mixtures thereof.
 15. The process of claim 1, wherein the nanoscale azo pigment particles are organic monoazo laked pigments comprising a diazonium component linked to a coupling component through an azo or hydrazone group, with a counterion.
 16. The process of claim 15, wherein a precursor to the diazonium component of the monoazo laked pigment is a compound of Formula (2):

where R₁, R₂, and R₃ independently represent H, a straight or branched alkyl group of from about 1 to about 10 carbon atoms, halogen, NH₂, NO₂, CO₂H, or CH₂CH₃; and FM represents SO₃H, —C(═O)—NH-Aryl-SO₃ ⁻ (where the aryl group can be unsubstituted or substituted with either halogens or alkyl groups having from about 1 to about 10 carbons), CO₂H, halogen, NH₂, or —C(═O)—NH₂, or is a compound of Formula (3):


17. The process of claim 16, wherein the precursor to the diazonium component is selected from the group consisting of the following compounds of Formula (2) wherein: FM is SO₃H, R₁ is CH₃, R₂ is H, and R₃ is NH₂, FM is SO₃H, R₁ is CH₃, R₂ is Cl, and R₃ is NH₂, FM is SO₃H, R₁ is Cl, R₂ is CH₃, and R₃ is NH₂, FM is SO₃H, R₁ is Cl, R₂ is CO₂H, and R₃ is NH₂, FM is SO₃H, R₁ is Cl, R₂ is CH₂CH₃, and R₃ is NH₂, FM is SO₃H, R₁ is Cl, R₂ is Cl, and R₃ is NH₂, FM is SO₃H, R₁ is H, R₂ is NH₂, and R₃ is H, FM is SO₃H, R₁ is H, R₂ is NH₂, and R₃ is CH₃, FM is SO₃H, R₁ is NH₂, R₂ is H, and R₃ is Cl, FM is SO₃H, R₁ is H, R₂ is H, and R₃ is NH₂, FM is SO₃H, R₁ is H, R₂ is NH₂, and R₃ is H, FM is SO₃H, R₁ is NO₂, R₂ is NH₂, and R₃ is H, FM is —C(═O)—NH-Phenyl-SO₃ ⁻, R₁ is NH₂, R₂ is CH₃, and R₃ is H, FM is CO₂H, R₁ is H, R₂ is H, and R₃ is NH₂, FM is Cl, R₁ is H, R₂ is H, and R₃ is NH₂, FM is NH₂, R₁ is CH₃, R₂ is H, and R₃ is H, FM is NH₂, R₁ is H, R₂ is CH₃, and R₃ is H, FM is —C(═O)NH₂, R₁ is NH₂, R₂ is CH₃, and R₃ is H, FM is —C(═O)NH₂, R₁ is H, R₂ is NH₂, and R₃ is H, FM is NH₂, R₁ is H, R₂ is H, and R₃ is H, FM is SO₂NHCH₃, R₁ is OCH₃, R₂ is NH₂, and R₃ is CH₃, and FM is CO₂CH₃, R₁ is H, R₂ is H, and R₃ is NH₂.
 18. The process of claim 15, wherein a precursor to the coupling component of the monoazo laked pigment is selected from the group consisting of β-naphthol and derivatives thereof, naphthalene sulfonic acid derivatives, pyrazolone derivatives, acetoacetic arylide derivatives, and benzimidazolone derivatives.
 19. The process of claim 15, wherein a precursor to the coupling component is selected from the group consisting of compounds of Formulas (4)-(8), wherein * denotes a point of coupling or attachment to the azo or hydrazone group:

where FM represents H, CO₂H, SO₃H, —C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted or substituted with either halogens, or alkyl groups having from about 1 to about 10 carbons, CO₂H, halogen, NH₂, —C(═O)—NH₂, substituted benzamides of the formula:

wherein groups R₂′ R₃′, R₄′ and R₅′ can independently be H, alkyl groups having from about 1 to 10 carbons, alkoxyl groups, hydroxyl or halogens, or NO₂; or benzimidazolone amides of the formula:

where FM represents SO₃H, CO₂H, —C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted or substituted with either halogens, or alkyl groups having from about 1 to about 10 carbons, CO₂H, halogens, NH₂, —C(═O)—NH₂ groups R₃ and R₄ independently represent H, SO₃H;

where FM represents SO₃H, CO₂H, —C(═O)—NH-Aryl-SO₃ ⁻ where the aryl group can be unsubstituted or substituted with either halogens, or alkyl groups having from about 1 to about 10 carbons, CO₂H, halogens, NH₂, —C(═O)—NH₂; R₁, R₂, R₃ and R₄ independently represent H, SO₃H, —C(═O)—NH-Phenyl,

where G represents CO₂H, straight or branched alkyl having from 1 to about 10 carbons atoms; and R₁′, R₂′, R₃′ and R₄′ independently represent H, halogens, SO₃H, nitro NO₂ or alkoxyl groups;

where R₁′ represents a straight or branched alkyl group having from 1 to about 10 carbon atoms, R₂′ represents

where each of R_(a), R_(b), and R_(c) independently represents H, a straight or branched alkyl group having from 1 to about 10 carbon atoms, OCH₃, or halogens.
 20. The process of claim 15, wherein the counterion is selected from the group consisting of metals, non-metals, and cations or anions based on either carbon, nitrogen or phosphorus.
 21. The process of claim 1, wherein the at least one functional group of the sterically bulky stabilizer is selected from the group consisting of sulfonate/sulfonic acid, (thio)carboxylate/(thio)carboxylic acid, phosphonate/phosphonic acid, ammonium and substituted ammonium salts, phosphonium and substituted phosphonium salts, substituted carbonium salts, substituted arylium salts, alkyl/aryl (thio)carboxylate esters, thiol esters, primary and secondary amides, primary and secondary amines, hydroxyl, ketone, aldehyde, oxime, hydroxylamino, enamines, porphyrins, (phthalo)cyanines, urethane, carbamate, substituted ureas, guanidines and guanidinium salts, pyridine and pyridinium salts, imidazolium and (benz)imidazolium salts, (benz)imidazolones, pyrrolo, pyrimidine and pyrimidinium salts, pyridinone, piperidine and piperidinium salts, piperazine and piperazinium salts, triazolo, tetraazolo, oxazole, oxazolines and oxazolinium salts, indoles, indenones, and mixtures thereof.
 22. The process of claim 1, wherein the sterically bulky stabilizer comprises at least one aliphatic hydrocarbon moiety.
 23. The process of claim 1, wherein the sterically bulky stabilizer is selected from the group consisting of the following compounds:

wherein each Z is independently H, a metal cation selected from the group consisting of Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, and B, or an organic cation selected from the group consisting of NH₄ ⁺ and NR₄ ⁺, where R is an organic group,

wherein each Z is independently H, OH, NH₂, NHR′, or OR′, where R′ is a C₁-C₆ alkyl group or C₆-C₁₄ aryl group,

wherein each Z is independently H, a metal cation selected from the group consisting of Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, and B, or an organic cation selected from the group consisting of NH₄ ⁺, NR₄ ⁺, and PR₄ ⁺, where R is an organic group, and m and n are integers representing repeating methylene units where m+n>1,

wherein each Z is independently H, a metal cation selected from the group consisting of Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, and B, or an organic cation selected from the group consisting of NH₄ ⁺, NR₄ ⁺, and PR₄ ⁺, where R is an organic group, and m and n are integers representing repeating methylene units where m+n>1 per branch,

wherein each Z is independently H, a metal cation selected from the group consisting of Na, K, Li, Ca, Ba, Sr, Mg, Mn, Al, Cu, and B, or an organic cation selected from the group consisting of NH₄ ⁺, NR₄ ⁺, and PR₄ ⁺, where R is an organic group, m is an integer representing repeating methylene units where m≧1 and n is 0 or 1

wherein m is an integer from 0 to about 12; R is H, CH₃, or (CH₂)_(n)CH₃ where n is an integer of from 0 to about 5,

wherein m is an integer from 0 to about 12; R is H, CH₃, or (CH₂)_(n)CH₃ where n is an integer of from 0 to about 5; Z is H or CH₃; X is Cl, Br, I, SO₄ ²⁻, MeSO₄ ⁻, O₃S-p-(C₆H₄)CH₃,

wherein n is an integer from 0 to about 10,

wherein n is an integer from 0 to about 25,

wherein n is an integer from 1 to about 30,

wherein m is an integer from 1 to about 30 and n is an integer from 1 to about 11,

wherein n is an integer from 1 to about 30,

wherein n is an integer from 1 to about 14, and

wherein n is an integer from 1 to about
 30. 24. The process of claim 1, further comprising adding a surfactant selected from the group consisting of rosin compounds; acrylic-based polymers; styrene-based copolymers; copolymers of α-olefins; copolymers of vinyl pyridine, vinyl imidazole, and vinyl pyrrolidinone; polyester copolymers; polyamide copolymers; and copolymers of acetals and acetates.
 25. The process of claim 1, wherein the non-covalent association between the organic pigment and the sterically bulky stabilizer compound is at least one of van der Waals' forces, ionic bonding, coordination bonding, hydrogen bonding, and aromatic pi-stacking bonding.
 26. The process of claim 1, wherein presence of the associated stabilizer limits an extent of particle growth and aggregation, to afford nanoscale-sized particles of azo pigments without clogging the microreactor or micromixer. 